Semiconductor device, liquid crystal display device, semicondutor film producing method, and semiconductor device producing method

ABSTRACT

This invention concerns with a semiconductor device which is characterized in that the device is provided with a thin film transistor  40  having a polycrystalline semiconductor layer  11 , the semiconductor layer  11  including a channel area  22 , highly doped drain areas  24, 17  positioned on both sides of the channel area  22  and LDD areas  18   a   , 18   b  positioned between the channel area  22  and the highly doped drain areas  24, 17  and lower in dopant density than the highly doped drain areas  24, 17 , wherein any diameter of the crystal  14  at least partly existing in the LDD area  18   b  is larger than the size of other crystals  15.

FIELD OF THE INVENTION

[0001] The present invention relates to a semiconductor device, a liquidcrystal display device, an EL display device, a method for fabricating asemiconductor thin film and a method for manufacturing the semiconductordevice.

BACKGROUND ART

[0002] A laser anneal method is generally known as a method of producinga semiconductor thin film for forming a semiconductor layer of a thinfilm transistor (hereinafter referred to as “TFT”). The laser annealmethod comprises the steps of forming an amorphous semiconductor film ora microcrystalline semiconductor film on a substrate made of glass orthe like, and irradiating the film with laser beams for crystallizationto give a polycrystalline semiconductor film. Usually this method iscalled a crystallization process.

[0003] Argon laser, KrF and XeCl excimer laser are generally used as alight source for laser beams to be employed in the crystallizationprocess. The TFT produced by the foregoing method is generally called alow-temperature poly Si-TFT since Si is mainly used as a semiconductorand the process is performed at a temperature below the melting point ofglass used as the substrate.

[0004] Conventional TFT liquid crystal display devices generally includea TFT having a semiconductor layer formed of amorphous silicon, and isprovided with a circuit member for driving the pixels which is of thetype having IC chips fixed to the periphery of an image plane. On theother hand, even a driving circuit can be produced by use of thelow-temperature poly Si-TFT using a TFT formed on a glass substrate.That is, a region outside an image plane can be reduced at an outerperiphery of a panel of a liquid crystal display device which isgenerally called a picture frame and a more elaborate dot-pitch liquidcrystal display device can be produced. Various kinds of semiconductorcircuits can be formed on a glass substrate by use of a low-temperaturepoly Si-TFT having improved performance. That is, the so-calledsystem-on-panel (SOP) can be realized. Moreover, with use of alow-temperature poly Si-TFT, an EL display device can be produced byswitching an EL display element.

[0005] However, the low-temperature poly Si-TFT poses the followingproblems.

[0006] (1) The crystals in a polycrystalline silicon provide asemiconductor device having a superior performance and high reliability.

[0007] (Method of Producing a Semiconductor Thin Film)

[0008] To achieve the foregoing objects, a method of producing asemiconductor thin film according to the invention is characterized bycomprising the steps of forming a heat-dissipating layer from a materialhigher in heat conductivity than the semiconductor thin film on a partof an amorphous or polycrystalline semiconductor thin film formed on asubstrate, and irradiating the semiconductor thin film withhigh-intensity light rays or laser beams to achieve crystallization.

[0009] According to this method of producing a semiconductor thin film,when the semiconductor thin film is melted by irradiation of intensivelight such as flash lamp or laser beams, heat is dissipated by theheat-dissipating layer in the vicinity of the heat-dissipating layer inthe semiconductor thin film, whereby the vicinity thereof is quicklycooled. The cooling rate is reduced as the heated part is far and farfrom the heat-dissipating layer. As a result, a temperature gradient isestablished in the semiconductor thin film when it is cooled so that thecrystal grows along the temperature gradient, i.e. along a direction inwhich the heated part is more and more away from the vicinity of theheat-dissipating layer, whereby a large size crystal is formed. A TFTproduced from the semiconductor thin film thus obtained is improved inmobility because of a crystal of larger size than conventional crystal,whereby the degradation of performance is alleviated.

[0010] Preferred specific examples of the procedure of forming aheat-dissipating layer are as follows.

[0011] A procedure comprising the steps of forming on the semiconductorthin film a film from a material higher in heat conductivity than thesemiconductor thin film; forming a resist mask by photolithography onthe film made of a material higher in heat conductivity; removing a partnot covered with the resist mask from the film made of a material higherin heat conductivity by an etching technique; and peeling the resistmask.

[0012] A procedure comprising the steps of forming a resist pattern byphotolithography; forming a film from a material higher in heatconductivity than the semiconductor thin film; and lifting off theresist pattern together with the film made of a material higher in heatconductivity.

[0013] A procedure of forming a film from a material higher in heatconductivity than the semiconductor thin film by vapor deposition orsputtering using a mask having openings.

[0014] In any of these procedures, the heat-dissipating layer can beeasily formed, resulting in an increase of productivity.

[0015] The heat-dissipating layer can be formed at a position in contactwith the semiconductor thin film and may be positioned on or under thesemiconductor thin film.

[0016] Another method of producing a semiconductor thin film accordingto the invention is characterized by comprising the step of irradiatingthe thin film with high-intensity light rays or laser beams at one ormore pulses over a specified range of the substrate in a fixed state ofpositional relationship between the substrate and a light source. In thecase of scanning irradiation wherein pulse irradiation is executed whilethe substrate or a light source is moved at a specified pitch, thecrystal grows correspondingly to the irradiation position, so that thecrystal having a greater size than the pitch width in the scanningdirection will not grow. On the other hand, the crystal can grow to alarge size irrespectively of scanning pitch width by pulse irradiationin a fixed state of positional relationship between the substrate andthe light source. Irradiation is executed at a plurality of pulses overa specified range of the substrate, whereby the irregularity ofirradiation intensity at each pulse is levelled, and the crystallinesize and film quality of the semiconductor thin film are made uniform,so that the irregularity in the performance of TFT produced can bediminished.

[0017] High-intensity light rays or laser beams can be supplied with apulse laser device by scanning irradiation in which irradiation isperformed at a plurality of pulses over a specified range of substratewhile relatively changing the positional relationship between thesubstrate and the light source at a specified pitch.

[0018] A further method of producing a semiconductor thin film accordingto the invention is characterized by comprising the steps of forming aheat-dissipating layer on a part of a substrate, forming an amorphous orpolycrystalline semiconductor thin film on the substrate, and applyinghigh-intensity light rays or laser beams to the semiconductor thin filmto achieve crystallization, wherein the heat-dissipating layer is madeof a material higher in heat conductivity than the semiconductor thinfilm.

[0019] According to this method of producing a semiconductor thin film,the semiconductor thin film is formed after forming the heat-dissipatinglayer, that is, the heat-dissipating layer is formed under thesemiconductor thin film, so that the heat-dissipating layer need not beremoved in producing a TFT using the semiconductor thin film. Since theremoval of heat-dissipating layer can be saved, the heat-dissipatinglayer can be used as the alignment key in the course of producing a TFT.Examples of the procedure of forming the heat-dissipating layer underthe semiconductor thin film include the following.

[0020] A procedure comprising the steps of forming a heat-dissipatinglayer on a substrate; forming an undercoat film having insulatingproperties over the substrate in a manner to cover the heat-dissipatinglayer with the undercoat film; and forming an amorphous orpolycrystalline semiconductor thin film on the undercoat film,

[0021] A procedure comprising the steps of forming an undercoat filmhaving insulating properties on a substrate; forming theheat-dissipating layer on the undercoat film; forming another undercoatfilm having insulating properties over the undercoat film in a manner tocover the heat-dissipating layer with the other undercoat film; andforming an amorphous or polycrystalline semiconductor thin film on theother undercoat film.

[0022] A still further method of producing a semiconductor thin filmaccording to the invention is characterized by comprising the step ofapplying high-intensity light rays or laser beams to an amorphous orpolycrystalline semiconductor thin film formed on a substrate via anexposure mask to achieve crystallization, wherein the exposure maskincludes a lens member having a curved surface on least one of the topand underside surfaces to give rise to an inclining distribution oflight quantity applied to the semiconductor thin film.

[0023] According to this method of producing a semiconductor thin film,high-intensity light rays or laser beams are penetrated through the lensmember of the exposure mask, whereby an inclining distribution of lightquantity applied to the semiconductor thin film is established and atemperature distribution is given to the semiconductor thin filmaccording to the distribution of light quantity. Thereby the moltensemiconductor thin film initiates solidification and crystallization ata portion having the lowest temperature, i.e. at a portion having thesmallest irradiated light quantity. The crystal grows toward a portioninvolving a large quantity of irradiated light along an incliningtemperature gradient, finally developing into a crystal of large size.When a TFT is produced using this semiconductor thin film, the mobilityis increased and the degradation of performance is attenuated because oflarger size crystal than conventional crystals.

[0024] Preferred specific examples of the method of giving rise to theforegoing distribution of light quantity include the following.

[0025] Using an exposure mask having a lens member in the form of astrip or a circle in a plan view, a distribution of light quantity isestablished in a lengthwise direction of the strip or a diameterdirection of the circle.

[0026] When the lens member takes the form of a strip in a plan view,the crystal grows from a portion involving a small light quantity to aportion involving a large light quantity along a lengthwise direction ofthe strip. When the lens member takes the form of a circle in a planview, the crystal grows in a direction from the vicinity of the centerof the lens member to the periphery thereof from a small light quantityto a large light quantity in a direction from the vicinity of center ofthe lens member to the periphery thereof. When the lens member takes theform of a circle in a plan view, the crystallization is initiated at apoint, i.e., a definite position, so that the position of a large sizecrystal being formed can be advantageously controlled with a highaccuracy. Specific examples of the lens member in the form of a circlein a plan view include a concave lens wherein the internal wall surfaceof a concave portion formed at an underside surface of the exposure maskis substantially spherical.

[0027] The curved surface of the lens member is preferably formed bydepressing at least a part of the top and underside surfaces of theexposure mask, or may be formed by forming the lens member in a convexform in such manner that the convex part is given a greater thicknessthan other parts of the lens member.

[0028] An additional method of producing a semiconductor thin filmaccording to the invention is characterized by comprising the step ofirradiating an amorphous or polycrystalline semiconductor thin filmformed on a substrate with high-intensity light rays or laser beams viaan exposure mask to achieve crystallization, wherein the exposure maskis so configured that an inclining distribution of light quantityapplied to the semiconductor thin film is brought about by giving aphase distribution to the irradiated light quantity.

[0029] According to the above-described method of producing asemiconductor thin film, the inclining distribution of light quantityirradiated to the semiconductor thin film is established due tointerference of light resulting from the phase distribution so that atemperature distribution is set up in the semiconductor thin filmaccording to the distribution of light quantity. Thereby the moltensemiconductor thin film initiates solidification and crystallization ata portion involving the lowest temperature, i.e., a portion involvingthe smallest irradiated light quantity. Then the crystal grows toward aportion involving a large light quantity along an inclining temperaturegradient, finally developing into a crystal having a large size. Inproducing a TFT using this semiconductor thin film, the mobility isincreased and the degradation of performance is decreased due to thelarger size crystal than conventional crystals.

[0030] Preferred examples of the method of establishing the foregoingphase distribution include the following which facilitate establishing adistribution of light quantity.

[0031] Using an exposure mask made of a light transmitting materialwhich is partly different in thickness, a phase distribution is given tothe irradiated light rays according to the thickness distribution.

[0032] For example, a level difference is made by forming a concaveportion of cylindrical shape on an internal wall of an underside surfaceof the exposure mask, whereby a phase distribution can be given to theirradiated light. When a concave portion is made in a circular form in aplan view, the starting position of crystallization is definite as apoint so that advantageously the position of a large size crystal beingformed can be precisely controlled.

[0033] Another method of producing a semiconductor thin film accordingto the invention is characterized by comprising the step of applyinghigh-intensity light rays or laser beams to an amorphous orpolycrystalline semiconductor thin film formed on a substrate via anexposure mask to achieve crystallization, wherein the exposure mask isformed of a light-intercepting material and has a plurality of openingsby which an inclining distribution of light quantity applied to thesemiconductor thin film is established.

[0034] According to this method of producing a semiconductor thin film,an inclining distribution of light quantity applied to the semiconductorthin film is established by suitably determining the size, shape andarrangement of openings so that a temperature distribution is broughtabout in respect of the semiconductor thin film according to thedistribution of light quantity. Thereby the molten semiconductor thinfilm initiates solidification and crystallization at a portion of thefilm involving the lowest temperature, i.e., a portion involving thesmallest irradiated light quantity. Then the crystal grows along aninclining temperature gradient toward a portion involving a largequantity of irradiated light, finally developing into a crystal having alarge size. In producing a TFT using this semiconductor thin film, themobility is improved and the degradation of performance is mitigated dueto the larger size crystal than conventional crystals.

[0035] Preferred specific examples of the method of giving rise to thedistribution of light quantity include the following.

[0036] The foregoing distribution of light quantity is established alonga lengthwise direction of the strip area using an exposure mask having aplurality of openings such that a rate of openings per area unit isstepwise or continuously varied along a lengthwise direction of thestrip area.

[0037] The foregoing distribution of light quantity is established alonga diameter direction of the circular area using an exposure mask havinga plurality of openings such that a rate of openings per area unit isstepwise or continuously increased along a diameter direction from thecenter of the circular area to the periphery thereof.

[0038] When a rate of openings per area unit is varied along alengthwise direction of the strip area, the crystal grows from a portioninvolving a small light quantity along a lengthwise direction toward aportion involving a large light quantity. When the rate of hole area perarea unit is increased from the center of the circular area to theperiphery thereof in a diameter direction, the crystal grows from thecenter of the circular area to the periphery thereof. When thedistribution of light quantity makes an inclining change, the crystalgrows to a larger size. In the latter case, since the crystallization isinitiated at a definite position, i.e., a point, the position of a largesize crystal being formed can be advantageously controlled with a highaccuracy.

[0039] In the method of producing a semiconductor thin film, asemiconductor thin film may be formed after forming a porous insulatingfilm on a substrate, whereby the crystal of larger size can be obtained.

[0040] (Method of Producing a Semiconductor Device)

[0041] To achieve the foregoing objects, a method of producing asemiconductor device according to the invention is characterized bycomprising the steps of forming a heat-dissipating layer and analignment key on a part of an amorphous or polycrystalline semiconductorthin film formed on a substrate, the heat-dissipating layer being madeof a material higher in heat conductivity than the semiconductor thinfilm, irradiating the semiconductor thin film with high-intensity lightrays or laser beams for crystallization, and forming a gate electrodefilm on the semiconductor thin film, wherein the alignment key is usedat least in a photo procedure for forming a pattern of the gateelectrode at a specified position by etching a part of the gateelectrode film.

[0042] According to this method of producing a semiconductor device, thesemiconductor thin film is melted by emitting intensive light or laserbeams and heat is dissipated by the heat-dissipating layer in thevicinity of a part of the thin film having the heat-dissipating layer,whereby the vicinity is quickly cooled. The cooling rate is graduallylowered as the part of the film is more and more away from theheat-dissipating layer. As a result, a temperature gradient occurs inthe semiconductor thin film being cooled so that the crystal grows alongthe temperature gradient, i.e. along a direction of the part of the filmbecoming more distant from the vicinity of the heat-dissipating layer,finally developing into a crystal having a larger size.

[0043] In producing a TFT using the foregoing semiconductor thin film, adefect chiefly existing in a grain boundary is alleviated or removed,thereby leading to improvements in mobility and in other characteristicsof TFT because of larger size crystal than conventional crystals, sothat a semiconductor device with enhanced performance and higherreliability can be obtained. Specific methods of producing aheat-dissipating layer are referred to the aforesaid methods ofproducing a semiconductor thin film.

[0044] In addition, an alignment key is formed in the semiconductor thinfilm so that using the alignment key, a gate electrode can be formed,whereby a TFT can be formed at the desired position corresponding to thelarge size crystal.

[0045] Even if a large size crystal was conventionally formed in asemiconductor thin film, means for producing a TFT according to thecrystal was unavailable, so that the presence or absence of grainboundary or the number of grain boundaries were variable in LDD oroffset areas and a channel area, resulting in irregularities of TFTperformance. However, according to the above-mentioned method ofproducing a semiconductor device, a TFT or a part of TFT structure canbe produced in the position of large size crystal instead of theposition of grain boundary. Consequently the above-mentioned problem canbe alleviated.

[0046] It is preferred to form an alignment key in the same steptogether with a heat-dissipating layer simultaneously.

[0047] Another method of producing a semiconductor device according tothe invention is characterized by comprising the steps of forming analignment key on a part of a substrate, forming an amorphous orpolycrystalline semiconductor thin film on the substrate and on thealignment key, irradiating the semiconductor thin film withhigh-intensity light rays or laser beams for crystallization, andforming a gate electrode film on the semiconductor thin film, whereinthe alignment key is formed of a material higher in heat conductivitythan the semiconductor thin film and is used at least in a photoprocedure for forming a pattern of the gate electrode at a specifiedposition by etching a part of the gate electrode film.

[0048] According to this method of producing a semiconductor device, theperformance of TFT can be enhanced and a semiconductor device can beobtained with improved performance and high reliability as describedabove. Moreover, since the alignment key functions as a heat-dissipatinglayer, the productivity can be increased.

[0049] A further method of producing a semiconductor device according tothe invention is characterized by comprising the steps of applyinghigh-intensity light rays or laser beams to an amorphous semiconductorthin film formed on a substrate via an exposure mask to accomplishcrystallization in a state wherein a distribution of light quantity hasbeen established, forming an alignment key, and forming a gate electrodefilm on the semiconductor thin film, wherein the alignment key is formeddue to the difference of color between a polycrystalline silicon areaand an amorphous silicon area created in the semiconductor thin film byshutting off a part of penetrated light rays with an exposure mask, andwherein the alignment key is used at least in a photo procedure forforming a pattern of the gate electrode at a specified position byetching a part of the gate electrode film.

[0050] According to the above-mentioned method of producing asemiconductor device, a distribution of light quantity applied to thesemiconductor thin film is set up, thereby establishing a temperaturedistribution in the semiconductor thin film according to thedistribution of light quantity. As a consequence, the moltensemiconductor thin film initiates solidification and crystallization ata portion involving the lowest temperature, i.e., a portion entailingthe smallest irradiated light quantity. Then the crystal grows toward aportion involving a large quantity of irradiated light, eventuallydeveloping into a crystal having a large size. In producing a TFT usingthis semiconductor thin film, the defect existing mainly in the grainboundary is alleviated or removed due to a larger size of the crystalthan conventional crystals, thereby leading to improvement in mobilityand other characteristics of TFT, so that a semiconductor device withenhanced performance and higher reliability can be obtained. Forspecific methods of establishing the distribution of light quantity, theaforesaid methods of producing a semiconductor thin film is referred to.

[0051] Since an alignment key is formed in the semiconductor thin film,a gate electrode can be produced using the alignment key, and a TFT canbe formed at the desired position with respect to the large sizecrystal. Consequently a TFT or a part of TFT structure can be producedin the position of large size crystal. Thus, the problem on theirregularities in performance of TFT can be alleviated.

[0052] The alignment key can be formed by applying light rays to thearea of the semiconductor thin film corresponding to the key patternformed in the exposure mask to give a polycrystalline area and byshutting off the irradiated light rays around the area with the exposuremask to give an amorphous area. Or the exposure mask may be formed suchthat the amorphous area is formed with only the part corresponding tothe key pattern as a non-irradiation part and its periphery isirradiated with light rays to give a polycrystalline area. It isdesirable to form the amorphous area and the polycrystalline area in thesame layer of the semiconductor thin film.

[0053] A still further method of producing a semiconductor deviceaccording to the invention is characterized by comprising the steps offorming a gate electrode and an alignment key on a part of a substrate,forming an amorphous or polycrystalline semiconductor thin film on thegate electrode and on the alignment key, forming a heat-dissipatinglayer from a material higher in heat conductivity than the semiconductorthin film in a specified position of the semiconductor thin film usingthe alignment key and irradiating the semiconductor thin film withhigh-intensity light rays or laser beams for crystallization.

[0054] According to this method of producing a semiconductor device, alarge size crystal can be formed in accordance with the position of thegate electrode by forming a heat-dissipating layer using the alignmentkey, so that the large size crystal and TFT can be positionedaccurately. Therefore the foregoing performance of TFT can be increasedand a semiconductor thin film having improved performance and highreliability can be formed.

[0055] (Semiconductor Device)

[0056] To achieve the foregoing objects, the semiconductor device of theinvention is characterized in that the device is provided with a thinfilm transistor having a polycrystalline semiconductor layer, thesemiconductor layer including a channel area, highly doped drain areaspositioned on both sides of the channel area and LDD or offset areaspositioned between the channel area and the highly doped drain areas,the LDD or offset areas being lower in dopant density than the highlydoped drain areas or being free of dopant, and that any diameter of acrystal at least partly existing in the LDD or offset areas is largerthan that of other crystals. The term “size (of a crystal)” used hereinis a value obtained by measuring the longest size of the crystal in anoptional direction in a plan view.

[0057] When a current flows in a TFT, which is on, constituting thesemiconductor device, carriers moving at a high rate in the channel areamay be scattered on collision with a defect of crystals. This is called“hot carrier phenomenon. The scattered carriers strike againstneighboring weak bonds such as those of Si—H and cut the bonds intodangling bonds of Si. On formation of dangling bonds, other carriers arecaptured so that the TFT becomes extremely lower in electricalconductivity and mobility, and the performance of TFT is degraded.

[0058] The defects of crystals and bonds of Si—H concentratedly exist inthe vicinity of a grain boundary. When numerous grain boundaries existin the LDD or offset area on the drain side, the performance may beimpaired and the reliability may be degraded.

[0059] The grain boundaries existing in the LDD or offset areas can bereduced compared with conventional grain boundaries or can be totallyremoved by giving any larger diameter to a crystal at least partlyexisting in the LDD or offset areas than other crystals. Thereby theperformance and the reliability can be improved.

[0060] For example, the following cases fall under the above: a casewherein as shown in FIG. 30(a), a crystal C1 partly existing in an areaA representing the LDD or offset area is greater in size than anothercrystal C2 and a grain boundary B slightly exists in the area A, or acase wherein as shown in FIG. 30(b), a crystal C3 entirely inclusive ofthe area A is so greater in the size than the other crystal C4 that nograin boundary exists in the area A.

[0061] The other crystal referred to above for comparison of the size ispreferably one existing outside the LDD or offset area. That is,preferably any diameter of the crystal at least partly existing in theLDD or offset area is greater than other crystals existing in itsentirety outside the LDD or offset area (more preferably the othercrystal existing in the channel area).

[0062] When numerous grain boundaries exist in the vicinity of theboundaries between the channel area and the LDD or offset area on thedrain side, the performance is more degraded and the reliability aremore impaired. Therefore it is preferred that any diameter of a crystalat least partly existing in an area in the range of 0.5 μm or less onthe LDD or offset area side including the boundary, away from at leastone of the boundaries between the channel area and the LDD or offsetareas is greater than that of the other crystal. The area is preferably0.4 μm or less, more preferably 0.3 μm on the LDD or offset area sideincluding the boundary.

[0063] In this case, it is desirable that any diameter of a crystal atleast partly existing in said area is greater than any diameter of othercrystal existing in its entirety outside the LDD or offset area (morepreferably the other crystal existing in the channel area).

[0064] The present inventors conducted experiments and found that thereis a interrelation (as shown in FIG. 31) between the size of apolycrystalline silicon crystal and the TFT reliability. The boundarybetween the channel area and the LDD or offset area which constitutes aTFT is set to coincide with the center of the diameter of the crystal.The reliability is determined by conducting a resistance test in whichan on/off operation of gate voltage is repeated at 500 kHz for 1500hours by applying 5V voltage across a source and drain in TFT's havingan LDD area or an offset area, respectively to perform a switchingoperation at a frequency of several times and is expressed in terms of aratio of mobility before and after the test.

[0065] As apparent from the same drawing, when the crystalline size is0.6 μm or more, the reliability in any case of LDD area or offset areais 75% or more and is good. The more distant from the boundary betweenthe channel area and the LDD or offset area the grain boundary is, themore reliable the TFT is. The crystalline size is preferably 0.8 μm ormore, more preferably 1 μm or more.

[0066] Our review done thereafter on this matter shows that thereliability is adversely affected by the grain boundary existing in thevicinity of the area boundary on the side of LDD or offset area amongthe grain boundaries positioned on both sides of the area boundary. Thatis, an electrical field is high in the vicinity of the area boundary inthe LDD or offset area on the drain side, so that when the grainboundary exists in this position, hot carriers are likely to develop.Further the semiconductor layer tends to become broken starting from thegrain boundary. As a result, the TFT performance is degraded and thereliability is lowered in the case of switching operations continued fora long time or repeated many times.

[0067] Consequently, it is effective to keep the grain boundary at aspecific distance away from the foregoing area boundary on the side ofLDD or offset area among the grain boundaries located on both sides ofthe area boundary. This distance corresponds to half the crystallinesize in the aforesaid experiments and is preferably 0.3 μm or more, morepreferably 0.4 μm or more, most preferably 0.5 μm or more. If aconfiguration is so formed that the grain boundary does not exist in therange of 0.3 μm or less on the side of the LDD or offset area includingthe boundary away from at least one of boundaries between the channelarea and the LDD or offset area. Thereby the defect of causing a hotcarrier phenomenon in the vicinity is alleviated. Even if hot carrierstake place, dangling bonds chiefly responsible for the degradation ofperformance would not occur in view of a lesser number of weak bondssuch as those of Si—H. Moreover, the semiconductor layer is unlikely tobecome broken due to the defect, resulting in attenuated degradation ofTFT performance and in increased reliability.

[0068] The area boundary without a grain boundary is negligible if it ison a drain side. However, depending on the semiconductor device, thedrain and source may be exchanged for each other. In this case, it ispreferable to configure the device such that the grain boundary is notpresent in the area boundaries on both sides of drain and source.

[0069] In the semiconductor device, it is preferable to keep the grainboundary at a distance of 0.3 μm on the channel area side, away from theboundary between the channel area and the LDD or offset area. Thedistance is more preferably 0.4 μm or less, most preferably 0.5 μm orless. Consequently, the grain boundary is not present in the specifieddistance on the side of the channel area as well as in the specifieddistance on the side of the LDD or offset area of the area boundary, sothat the mobility is enhanced, the degradation of TFT performance isattenuated, and an increase in reliability is assured.

[0070] Another semiconductor device of the invention is characterized inthat the device is provided with a thin film transistor having apolycrystalline semiconductor layer, the semiconductor layer including achannel area, highly doped drain areas positioned on both sides of thechannel area and LDD or offset areas positioned between the channel areaand the highly doped drain areas, the LDD or offset areas being lower indopant density than the highly doped drain areas or being free ofdopant, wherein a grain boundary is not present at least in the LDD oroffset area on one side.

[0071] According to the foregoing semiconductor device, a grain boundaryis not present in the LDD or offset area on the drain side having a partwhich is high in electrical field so that the generation of hot carrierscan be suppressed, the degradation of TFT performance can be attenuatedand the reliability can be enhanced.

[0072] Further when the device is so configured that a grain boundary isnot present in the channel area, the mobility is improved, thedegradation of TFT performance can be lowered and the increase ofreliability is assured.

[0073] Furthermore, when the device is so configured that a grainboundary is not present in the highly doped drain area adjacent to theLDD area or offset area, the configuration is effective in reducing thecontact resistance of source or drain and substantially increasing anon-state current of TFT.

[0074] A further semiconductor device of the invention is characterizedin that the device is provided with a plurality of thin film transistorshaving a function in common, and that 50% (the fractional portion of thenumber is dropped) or more of the thin film transistors are theforegoing thin film transistors. The provision of 70% or more thereof ismore preferable and the provision of 90% or more is the most preferable.For example, in a liquid crystal display device or an EL display deviceas an example of the semiconductor device, the TFT's for controlling theoperation of each pixel, for example, are 100 in number, and theabove-mentioned TFT's are preferably 50 or more in number.

[0075] According to this semiconductor device, the plurality of thinfilm transistors include the above-described thin film transistors at aspecified ratio or more which are sufficient to attenuate thedegradation of TFT performance and to increase the reliability. Thus,stable performance is assured.

[0076] Preferably each of the above-mentioned semiconductor devices hasan insulating undercoat film between the substrate and the semiconductorlayer. Preferably the foregoing undercoat film includes a porous layercontaining pores of 0.1 to 2 μm in average pore size. The pore size canbe measured by observation under an electron microscope typically havinga cross section SEM·TEM.

[0077] The undercoat film including the porous layer formed between thesubstrate and the semiconductor layer is effective in accelerating thecrystal growth of the semiconductor layer. However, the porous layercontaining pores with an excessively large pore size fails toeffectively prevent diffusion of dopant from the substrate to thesemiconductor layer. In the case of allowing the TFT to executeswitching operation continuously for a prolonged time or repeatedly manytimes, the threshold value (Vt) of gate voltage in change-over from anoff operation to on operation is shifted. When large hollow pores existat an interface between the channel area and the LDD area, TFT can notfunction, resulting in a lower yield.

[0078] From the viewpoint of the above, the porous layer has hollowpores of preferably 0.01 to 2 μm, more preferably 0.05 μm to 0.1 μm inaverage pore size. Thereby not only an increase of grain size in thesemiconductor layer is achieved but also the percent defective of TFT islowered. Further the threshold value (Vt) of gate voltage in change-overfrom an off operation to on operation can be prevented from shifting inthe case of allowing the TFT to execute switching operation continuouslyfor a prolonged time or repeatedly many times.

[0079] The insulating undercoat film formed between the substrate andthe semiconductor layer may be preferably so configured as to include aporous layer containing pores 0.001 μm to 2 μm in average pore size anda denser layer formed on the porous layer than the porous layer.

[0080] According to this semiconductor device, the diffusion of dopantcan be prevented by the dense layer constituting the undercoat film, thepercent defective of TFT is lowered and the threshold value (Vt) of gatevoltage in change-over from an off operation to on operation can beprevented from shifting in allowing the TFT to execute switchingoperation continuously for a prolonged time or repeatedly many times. Inaddition, the crystal growth in the semiconductor layer is acceleratedby the porous layer constituting the undercoat film.

[0081] A still further semiconductor device of the invention ischaracterized in that the thin film transistor is formed in the vicinityof the pattern in the specified shape which is made of a material higherin heat conductivity than the semiconductor layer.

[0082] According to this semiconductor device, a large size crystal canbe easily formed in the semiconductor layer by the pattern in thespecified shape which is made of a material higher in heat conductivitythan the semiconductor layer.

[0083] The above-mentioned pattern is preferably formed between thesubstrate and the semiconductor layer, and is more preferably coveredwith the insulating undercoat film formed between the substrate and thesemiconductor layer. Thereby the pattern can be used as an alignment keyin the photo procedure in production of a semiconductor device.

[0084] The undercoat film may be composed of a first undercoat film(upper undercoat film) and a second undercoat film (lower undercoatfilm). The above-described pattern may be formed between the firstundercoat film and the second undercoat film. In this case, the firstundercoat film may be preferably made thinner than the second undercoatfilm so that the heat conductivity is increased and a larger sizecrystal can be formed. The above-mentioned pattern is formed ofpreferably a metal film, and can be formed in the vicinity of the drainarea, channel area or source area of the semiconductor layer.

[0085] It is possible to produce a semiconductor device wherein thesemiconductor thin film on the periphery of the pattern contains acrystal of longer size than in other parts. The pattern may be providedin contact with the semiconductor thin film. It is also possible toproduce a semiconductor device wherein the crystals in the semiconductorthin film positioned immediately on or under the pattern have a shortersize than the crystal in the semiconductor thin film on the periphery ofthe pattern.

[0086] The foregoing semiconductor devices can be produced, for example,by the above-mentioned method of producing a semiconductor device. Forexample, the following semiconductor devices can be produced by theabove-mentioned method of producing a semiconductor device.

[0087] A semiconductor device which is characterized in that the deviceis provided with a thin film transistor having a semiconductor layerformed on a substrate, the semiconductor layer including a channel area,highly doped drain areas positioned on both sides of the channel areaand LDD or offset areas positioned between the channel area and thehighly doped drain areas, the LDD or offset areas being lower in dopantdensity than the highly doped drain areas or being free of dopant, andthat any diameter of the crystal existing in the vicinity of theboundary between the channel area and the LDD or offset areas is largerthan that in other areas.

[0088] A semiconductor device which is characterized in that the deviceis provided with a thin film transistor having a semiconductor layerformed on a substrate, the semiconductor layer including a channel area,and highly doped drain areas positioned on both sides of the channelarea, and that the size of crystal existing in the vicinity of theboundary between the channel area and the highly doped drain area islarger than in other areas.

[0089] A semiconductor device which is characterized in that the deviceis provided with a thin film transistor having a semiconductor layerformed on a substrate, the semiconductor layer including a channel area,highly doped drain areas positioned on both sides of the channel areaand LDD or offset areas positioned between the channel area and thehighly doped drain areas, the LDD or offset areas being lower in dopantdensity than the highly doped drain areas or being free of dopant, andthat the size of the crystal in the source area is different from thatof the crystal in the LDD or offset area, or the size of the crystal inthe source area is different from that of the crystal in the drain area(e.g., the size of the crystal in the drain area is smaller than in thesource area).

[0090] A semiconductor device which is provided with a thin filmtransistor having a semiconductor layer formed on a substrate, and thatone grain boundary exists in one channel area of the semiconductorlayer.

[0091] Another semiconductor device of the invention is characterized inthat the thin film transistor has a semiconductor layer formed of apolycrystalline semiconductor thin film and a pattern in the specifiedshape formed of an amorphous semiconductor thin film.

[0092] According to this semiconductor device, the pattern in thespecified shape can be used as an alignment key in a photo procedure inproduction of the semiconductor device. Preferably the polycrystallinesemiconductor thin film and the amorphous semiconductor thin filmconstitutes the same layer.

[0093] The foregoing semiconductor devices include, for example, aliquid crystal display device and an EL display device which allow eachpixel to operate by feeding a voltage via the semiconductor deviceincluding a plurality of thin film transistors. In this case, thelifetime can be prolonged to an extent to which point defects or linedefects appear in images. Further, the accuracy of fine images anduniformity of image luminance can be improved, and the yield andreliability can be increased. In an EL display device, pixels and adriving circuit can be produced using the above-mentioned TFT, and canbe driven and can perform image display with such TFT. The EL displaydevice includes both of an inorganic EL display and an organic ELdisplay.

BRIEF DESCRIPTION OF THE DRAWINGS

[0094]FIG. 1 is a perspective view showing a substrate having anamorphous silicon film formed thereon in a method of producing asemiconductor thin film as an embodiment 1 of the present invention.

[0095]FIG. 2 is a schematic configuration diagram of a laser annealdevice to be used in the method of producing a semiconductor thin film.

[0096]FIG. 3 is a sectional view showing a state of the film afterapplication of laser beams in the method of producing a semiconductorthin film.

[0097]FIG. 4 is a sectional view showing a state wherein aheat-dissipating layer and an alignment key have been formed on theamorphous silicon film in the method of producing a semiconductor thinfilm.

[0098]FIG. 5(a) is a sectional view showing a state of a primaryirradiation step in the method of producing a semiconductor thin film,and FIG. 5(b) shows a temperature distribution of the silicon filmaccording to the primary irradiation step.

[0099]FIG. 6 is a sectional view showing a state in which a large sizecrystal has been formed in the silicon film due to the presence of theheat-dissipating layer in the method of producing a semiconductor thinfilm.

[0100]FIG. 7 is a plan view showing the vicinity of the large sizecrystal.

[0101]FIG. 8 is a sectional view for describing a step of removing theheat-dissipating layer in the method of producing a semiconductor thinfilm.

[0102]FIG. 9 is a plan view showing the shape of the large size crystalwhen the shape of the heat-dissipating layer has been changed in themethod of producing a semiconductor thin film.

[0103]FIG. 10 is a sectional view for describing a step of producing aTFT constituting the semiconductor device in the method of producing asemiconductor device as the embodiment 1 of the present invention.

[0104]FIG. 11(a) is a sectional view showing the positional relationshipbetween the large size crystal and the TFT in the semiconductor deviceas the embodiment 1 of the invention and FIG. 11(b) is a plan viewshowing the same. FIG. 12 is a perspective view schematically showing aliquid crystal display device partly cut away as an example of thesemiconductor device according to the embodiment 1 of the invention.

[0105]FIG. 13 is a schematic plan view showing a part of the liquidcrystal display device.

[0106]FIG. 14 is a sectional view showing an EL element of the ELdisplay device as an example of the semiconductor device in theembodiment 1 of the invention.

[0107]FIG. 15 is a circuit diagram showing a part of the EL displaydevice.

[0108]FIG. 16 is a sectional view for describing a step of forming aheat-dissipating layer and an alignment key(s) in the method ofproducing a semiconductor thin film as an embodiment 3 of the invention.

[0109]FIG. 17 is a sectional view showing a state wherein an offset areaof TFT constituting the semiconductor device is formed in the method ofproducing a semiconductor device as an embodiment 3 of the invention.

[0110]FIG. 18 is a sectional view showing a state wherein a secondundercoat film having a porous layer is formed on a first undercoat filmin the method of producing a semiconductor thin film as an embodiment 7of the invention.

[0111]FIG. 19 is a perspective view showing a state wherein the primaryirradiation step is being carried out using an exposure mask in themethod of producing a semiconductor thin film as an embodiment 8 of theinvention.

[0112]FIG. 20(a) is a sectional view schematically showing a statewherein a distribution of light quantity is established on a siliconfilm by the primary irradiation step using the exposure mask shown inFIG. 19, and FIG. 20(b) is a schematic view of the distribution of lightquantity.

[0113]FIG. 21 is a plan view of the exposure mask to be used in themethod of producing a semiconductor thin film as an embodiment 9 of theinvention.

[0114]FIG. 22(a) is a sectional view schematically showing a statewherein a distribution of light quantity is established on a siliconfilm by the primary irradiation step using the exposure mask illustratedin FIG. 21 and FIG. 22(b) is a diagram schematically showing thedistribution of light quantity.

[0115]FIG. 23 is a perspective view showing a state wherein the primaryirradiation step is being carried out using an exposure mask in themethod of producing a semiconductor thin film as an embodiment 10 of theinvention.

[0116]FIG. 24(a) is a sectional view schematically showing a statewherein a distribution of light quantity is established on a siliconfilm by the primary irradiation step using the exposure mask shown inFIG. 19 and FIG. 24(b) is a diagram schematically showing thedistribution of light quantity.

[0117]FIG. 25(a) is a side view of the exposure mask to be used in themethod of producing a semiconductor thin film as an embodiment 11 of theinvention, and FIG. 25(b) is a perspective view thereof.

[0118]FIG. 26(a) is a sectional view schematically showing a statewherein a distribution of light quantity is established on a silica filmby the primary irradiation step using the exposure mask shown in FIG. 25and FIG. 26(b) is a diagram schematically showing the distribution oflight quantity.

[0119]FIG. 27 is a plan view of the exposure mask to be used in themethod of producing a semiconductor thin film as an embodiment 12 of theinvention.

[0120]FIG. 28 is a sectional view for describing a step of producing aTFT constituting the semiconductor device in the method of producing asemiconductor device as an embodiment 13 of the present invention.

[0121]FIG. 29 is a sectional view for describing the step of producing aTFT constituting the semiconductor device in the method of producing asemiconductor device as an embodiment 14 of the present invention.

[0122]FIG. 30 shows, by way of example, a positional relationshipbetween the LDD or offset area and the crystals in the semiconductordevice of the invention.

[0123]FIG. 31 is a diagram showing an interrelationship between the sizeof silicon crystal and the reliability of TFT.

BEST MODE OF CARRYING OUT THE INVENTION

[0124] The embodiments of the invention will be described with referenceto the accompanying drawings.

EMBODIMENT 1

[0125] (Semiconductor Thin Film)

[0126] First, a method of producing a semiconductor thin film will bedescribed. The producing methods in this embodiment and in the otherembodiments can be carried out in the same manner relating tosemiconductor thin films made of, e.g., GaAs, Ge, SiGe, SiGeC or thelike. Nevertheless, the methods will be discussed mainly relating tosilicon (Si) generally used these days.

[0127] As shown in FIG. 1, an undercoat film 2 formed of SiO₂ having athickness of 300 nm is formed on a substrate 1, e.g., by a TEOS-CVDmethod in order to prevent diffusion of dopants from the substrate 1.The thickness of the undercoat film 2 is not limited to 300 nm but canbe set in a broad range. The substrate 1 is made of glass in thisembodiment, and may be composed of plastics or may be films. Siliconnitride films or the like can be used as the undercoat film 2. Noproblem would be posed if a SiO₂ film or silicon nitride film has athickness of 200 nm or more. The film thickness of less than 200 nmwould permit diffusion of dopants from the substrate 1 through a siliconlayer 9 and would be likely to raise a problem on Vt shift of TFTperformance and the like.

[0128] Subsequently an amorphous silicon film 3 is formed on theundercoat film 2 by a plasma CVD method. In forming the amorphoussilicon film 3, a reduced pressure CVD method or sputtering may beemployed. The thickness of the amorphous silicon film 3 may be in therange of preferably 30 to 90 nm. The thickness of 50 nm was adopted inthis embodiment.

[0129] Next, to remove hydrogen from the amorphous silicon film 3 thusformed, the film 3 was dehydrogenated by heat treatment at 450° C. for 1hour. However, dehydrogenation is not required when executing afilm-forming method such as sputtering or the like which would entailthe incorporation of little or no hydrogen into the amorphous siliconfilm 3.

[0130] Then, pre-irradiation of laser beams is performed with a laseranneal device (ELA device) 6 as shown in FIG. 2. In this embodiment,laser beams are applied repeatedly 10 times (10 pulses) to one positionwhile moving the substrate on a stage (not shown) using XeCl excimerlaser (wavelength 308 nm). Optionally laser beams may be applied bymoving a laser irradiator on a fixed substrate instead of moving asubstrate.

[0131] The laser beams for crystallization are absorbed into the siliconfilm for generation of heat. Thus, it is necessary to employ laser beamshaving a short wavelength of 500 nm or less. The shorter the wavelengthof laser is, the higher the absorption efficiency is. Hence such laseris preferable. In the invention, crystallization is achieved with use ofXeCl excimer laser (308 nm in wavelength). Useful laser beams includethose in the range of 500 nm or less in wavelength. For example, ArF,KrF or like excimer laser, Ar laser and the like are employed. Althougha pulse laser is used in the description of this embodiment, laser beamsof continuous waver (CW) are also employable. In this case, the numberof pulses in the following description is made to correspond to theirradiation time.

[0132] When laser beams 7 are applied to the amorphous silicon film 3,the beams 7 are applied at an energy density of about 160 mJ/cm² or moreat room temperature to bring about crystallization, whereby apolycrystalline silicon film 11 is successively formed in an arrowdirection. The irradiation area is necessarily determined to calculatethe energy density. In the specification, the irradiation area wasdetermined by measuring a distribution of laser intensity, andcalculating the area of a space defined by linking the positions atwhich ½ the highest intensity was exhibited.

[0133] In this pre-irradiation step, a highly membraneouspolycrystalline silicon film 11 is not needed. Crystals of small sizeare unlikely to develop crystalline defects in the step of forming asilicon film containing crystals of large size which is executed later.For example, the amorphous silicon film 3 is crystallized by applicationof laser beams at a relatively low intensity of 170 to 280 mJ/cm². Inthis embodiment, laser beams are applied at 250 mJ/cm², giving apolycrystalline film 11 having crystals with a size of 30 nm or less asshown in FIG. 3.

[0134] Then, a heat-dissipating layer 4 and alignment keys 5 are formedon the polycrystalline silicon film 11 as shown in FIG. 8(a) using amask for forming a pattern. In this embodiment, the film is formed byvapor deposition but can be formed also by means such as sputtering.

[0135] The heat-dissipating layer 4 is formed of a material higher inheat conductivity than the polycrystalline silicon film 11. Suitablematerials are substantially all of metals and alloys thereof includingAl, Ti, Ni, Cr, Ti, Mo, W, Cu, Au, Ag, Pt, Ta, In and the like. Metaloxides such as ITO (InTio) which are high in heat conductivity can beused. This matter can apply to the following embodiments. In thisembodiment, the heat-dissipating layer 4 and alignment keys 5 were madeof molybdenum-tungsten alloy (MoW). In this embodiment, theheat-dissipating layer 4 takes a rectangular shape in a plan view andmay assume other shapes including a triangle, circle and ellipse.

[0136] Thereafter the primary irradiation step is executed using thelaser anneal device (ELA device) again as shown in FIG. 5(a). A lowerlimit of laser intensity in the primary irradiation step is higher thanin the above-mentioned pre-irradiation step. An upper limit of laserintensity is lower than a value at which the polycrystalline siliconfilm 11 initiates change of properties or vaporization. Morespecifically, the intensity of laser beams 7 is in the range of 280mJ/cm² to 420 mJ/cm² and is 380 mJ/cm² in this embodiment. Further theupper limit and the lower limit of laser intensity are preferablysubstantially proportional to the thickness of the silicon film 3. Theintensity is preferably in the range which fulfils a relationship of3.78 Ta+138≦El≦4.54 Ta+153 wherein Ta is the thickness (nm) of thesilicon film 3 and El is an intensity density of laser beams.

[0137] With an increase in the frequency of applying laser beams 7 toone position, the irradiation intensity becomes more stable, and thecrystal size and film quality become uniform so that the performance ofTFT to be produced later is stabilized. On the other hand, the increasedfrequency results in a prolonged irradiation time and in decrease ofproductivity. From this point of view, a preferred frequency ofirradiation of laser beams 7 to one position is in the range of about 10to about 30 times. In this embodiment, laser beams are applied to oneposition at a frequency of 20 times while moving the substrate at asuitable pitch. When irradiation is performed at a frequency of 100times or more at one position, the mobility in the case of producing aTFT is increased to about 1.5 times as high as at a frequency of 20times.

[0138] The silicon film 11 which has become polycrystalline in thepre-irradiation step is melted again by the primary irradiation step. Inthe vicinity of the heat-dissipating layer 4 on the molten silicon film11, heat is transferred in an arrow direction 12 toward theheat-dissipating layer 4 which is higher in heat conductivity than thesilicon film 11 so that the film 11 is quickly cooled. The cooling ratebecomes increasingly lower in a position far and far from theheat-dissipating layer 4. As shown in FIG. 5(b), a temperature gradientis brought about in the vicinity of the heat-dissipating layer 4 after aspecified lapse of time. At a part of the film where the temperaturegradient is established, crystallization proceeds from a low temperatureside to a high temperature side. As a result, a large size crystal 14 isformed in the silicon film 11 in the vicinity of the heat-dissipatinglayer 4 as shown in FIG. 6 while smaller size crystals 15 are formed ata part of the film 11 far away from the heat-dissipating layer 4. Aregion 13 under the heat-dissipating layer 4 is not melted and retainsthe same state as the crystallization state in the pre-irradiation step,because the region 13 is not irradiated with laser beams due to thepresence of the heat-dissipating layer 4 serving as a mask.

[0139] The shape of the large size crystal 14 in a plan view is shown inFIG. 7. The size of the crystal was measured under an interatomic forcemicroscope (ATFM) and a transmission electron microscope (TEM). It wasfound that a size a in a lengthwise direction, i.e., in a direction ofthe temperature gradient brought about, was 1 μm and a size b in awidthwise direction, i.e., a direction orthogonal to the lengthwisedirection on a plane was 0.5 μm. The crystal size is expressed in termsof a maximum distance between grain boundaries in respective directions.A significant defect was not found in the crystal 14.

[0140] The size of small size crystals 15 on the periphery of the largesize crystal 14 was measured in the same manner as above and was 100 nmor less which was substantially the same as the size of crystals in thepolycrystalline silicon film formed in the conventional irradiationstep. As described above, at the primary irradiation step in thisembodiment, the size of the crystal in the silicon film can beselectively increased in the vicinity of the heat-dissipating layer 4,namely in a specified position, resulting in improvement of filmquality.

[0141] Then, a step of removing the heat-dissipating layer 4 is carriedout. As shown in FIG. 8(a), the alignment keys 5 are formed on thepolycrystalline silicon film 11 together with the heat-dissipating layer4. First, a protective film 16 such as a resist is formed on thealignment keys 5 and solidified by drying (FIG. 8(b)). Thereafter theheat-dissipating layer 4 is removed by dry etching or wet etching (FIG.8(c)). Finally the protective film 16 is removed with a release agent(FIG. 8(d)). Thereby the heat-dissipating layer 4 is removed while thealignment keys are left, whereby the semiconductor thin film iscompleted. Numerous dangling bonds are formed in the polycrystallinesilicon film 11. The dangling bonds of silicon atoms are terminated withhydrogen, e.g., by being left in hydrogen plasma at 450° C. for 2 hours.The concentration of hydrogen is, for example, about 2×10²⁰ atom·cm³.

[0142] In the production of the semiconductor thin film in thisembodiment, the heat-dissipating layer 4 is formed in a rectangularshape in a plan view, preferably in a triangular shape in a plan view.Because of the heat-dissipating layer 4 formed in this shape, crystalgrowth is initiated from the top of the heat-dissipating layer 4 afterthe primary irradiation step. As shown in FIG. 9, the large size crystal14 is made to assume a substantially fan-like shape in a plan view. Inthis case, the starting point of crystal growth is a point and thusdefinite, the positioning of a large size crystal and a TFT can beeasily achieved in the course of producing a TFT to be described later.

[0143] (Semiconductor Device)

[0144] Next, a method of producing a thin film transistor (TFT)constituting a semiconductor device will be described. First, using thesemiconductor thin film formed by the above-mentioned method as shown inFIG. 10 (a), the polycrystalline silicon film 11 having the large sizecrystal 14 is subjected to a photo procedure or etching procedure withthe alignment keys 5. After patterning in the form of an island, a gateinsulating film 19 is formed of silicon oxide. The alignment keys 5 areused for positioning the mask in the photo procedure to be describedbelow. The gate insulating film 19 can be formed by subjecting a film ofSiO₂, e.g., to plasma CVD using TEOS. The film 19 requires a thicknessof, e.g., 100 nm. The film 19 can be also formed, e.g., by reducedpressure CVD, remote plasma CVD, normal pressure CVD, ECR-CVD or thelike or by a high pressure oxidation or plasma oxidation.

[0145] As shown in FIG. 10(b), a gate electrode 20 is formed on the gateinsulating film 19. The gate electrode 20 can be produced, e.g., by amethod comprising forming a molybdenum-tungsten alloy film bysputtering, effecting a photo procedure using a photo mask for a gateelectrode, and patterning the film in the specified shape by etching.Other materials for forming a gate electrode include a highly pure Aland an Al material prepared by mixing Al with at least one speciesselected from Si, Cu, Ta, Sc, Zr and the like.

[0146] A mask (not shown) to be used in the photo procedure can bepositioned by the alignment keys 5. A gate electrode 20 is set in thevicinity of the large size crystal 14 of polycrystalline silicon film11. More specifically, the gate electrode 20 is formed in such mannerthat an end of the gate electrode 20 on the drain side (right side inthe drawing) coincides with the center of the large size crystal 14.

[0147] Then, an ion doping step is carried out as shown in FIG. 10(c).First, phosphorus in a low concentration is injected into the siliconfilm 11 using the gate electrode 20 as a mask by an ion doping device.Thereby a channel area 22 is formed immediately under the gate electrode20 in the silicon film 11. P-channel or n-channel transistors can beselectively produced by selective use of dopants other than phosphorusincluding boron or arsenic as an acceptor and aluminum as a donor. ACMOS circuit can be formed on the substrate.

[0148] Then, after forming a resist pattern over the gate electrode 20and in the range of 2 μm away from both ends thereof by a photoprocedure, highly concentrated phosphorus is injected into the siliconfilm 11 by an ion doping device using the resist pattern as a mask. As aresult, highly doped drain areas are formed at portions not covered withthe resist pattern on both sides of the channel area 22 in the siliconfilm 11. The highly doped drain areas are represented by a source area24 and a drain area 17. LDD areas 18 a, 18 b which are lower in dopantdensity than the highly doped drain areas reside between the channelarea 22 and the highly doped drain area 24, and between the channel area22 and the highly doped drain area 17, respectively.

[0149] Thereafter the resist is removed. The injected dopants areactivated by heat treatment or other means. As to activation of injectedions, an annealing procedure need not be additionally executed becauseof occurrence of self activation by the hydrogen injected together. Toassure the activation, however, the film may be locally heated by annealat 400° C. or higher, irradiation of excimer laser beams, RTA (rapidthermal anneal) or the like.

[0150] Thereafter, an interlaminar insulating layer 21 of silicon oxideis formed all over as shown in FIG. 10(d). The interlaminar insulatinglayer 21 can be formed by subjecting a film of SiO₂, e.g., to a plasmaCVD using TEOS, or of course, by other methods, e.g., AP-CVD(atmospheric pressure CVD) method, LTO (low temperature oxide), ECR-CVDor the like. The interlaminar insulating layer 21 may be formed ofsilicon nitride, tantalum oxide, aluminum oxide or the like, or may be alaminate of thin films made of these materials.

[0151] Then, contact holes are formed by etching in the interlaminarinsulating layer 21 and the gate insulating film 19 to reach the sourcearea 24 and the drain area 17 of the polycrystalline silicon film 11.Then, a titanium film or an aluminum zirconium alloy film is sputteredonto the contact holes and patterned in the specified shape by etchingto make a source electrode 23 a and a drain electrode 23 b. The sourceelectrode 23 a and the drain electrode 23 b may be formed of othermaterials than the aluminum zirconium alloy, such as aluminum (Al),tantalum (Ta), molybdenum (Mo), chrome (Cr), titanium (Ti) or likemetals or alloys thereof, or poly-Si containing a large quantity ofdopants. A transparent electroconductive layer of poly-Si Ge alloy, ITOor the like may be used.

[0152] By the foregoing process, an n-type TFT 40 as shown in FIG. 11(a)is completed. In need of a p-type TFT, B doping procedure is executedinstead of injecting phosphorus.

[0153] In the TFT of this embodiment, the gate electrode 20 is formed insuch a manner that the end of the gate electrode 20 on the drain sidecoincides with the center of the large size crystal 14. Consequently asingle crystal exists, as shown in FIG. 11, in the range (range shownwith a net lines in the drawing) of 0.5 μm away from both sides of theboundary between the LDD area 18 b on the drain side and the channelarea 22 wherein a grain boundary B is absent. Therefore the degradationof TFT due to generation of hot carriers can be prevented and thereliability can be increased. In the production of a semiconductor thinfilm, the heat-dissipating layer 4 is formed not only in the vicinity ofthe drain area 17 but also in the vicinity of the source area 24 tothereby avoid the presence of grain boundary B in the vicinity of theboundary between the LDD area 18 a on the source side of TFT 40 and thechannel area 22 (e.g. the range of 0.5 μm away from both sides), wherebythe performance and reliability can be further improved.

[0154] The mobility of the TFT 40 in this embodiment was measured andwas found to be 180 cm²/V.s. The mobility was significantly enhancedcompared with the mobility of 100 cm²/V.s of conventional TFT's producedusing the semiconductor thin film formed without forming aheat-dissipating layer 4. A reliability test was carried out byrepeating the switching operation several times, more specifically, anon/off operation of gate voltage at 500 kHz for 1500 hours at 5V voltageapplied across the source and drain. The mobility of the aboveconventional TFT was decreased by about 50% from the initial mobilityvalue, whereas the mobility of TFT 40 in this embodiment was 85% orhigher of the initial value, which means that the degradation ofperformance due to switching was reduced. The same reliability test wasconducted under the same conditions in respect of the embodimentsdescribed below.

[0155] (Liquid Crystal Display Device)

[0156] A liquid crystal display device having TFT's produced by theforegoing methods will be described. As shown in FIG. 12, a liquidcrystal display device 50 has a TFT array substrate 1 and an opposedsubstrate 31 arranged in opposition to each other.

[0157] The TFT array substrate 1 is provided with a plurality of TFT's40 on the side of an upper surface (on the side of the opposed substrate31) arrayed in a matrix with driving circuits 42, 44 on the periphery ofTFT 40. The opposed substrate 31 is a glass substrate (e.g. Corning Co.,Ltd., Lot No. 1737) serving as an insulating substrate and has a colorfilter 32 and a transparent electrode 33 on the side of an undersidesurface (on the side of the TFT array substrate 1). A liquid crystalmember 35 liquid-tightly containing a liquid crystal between orientatedfilms of polyamide or the like is interposed between the TFT arraysubstrate 1 and the opposed substrate 31. Polarizing plates 37, 39 arefixed to the surfaces in opposition to the opposed surfaces of the TFTarray substrate 1 and the opposed substrate 31.

[0158] One of pixel areas 56 in the TFT array substrate 1 is shown in anenlarged form in FIG. 13. A scanning line 52 and a data line 54 arearranged in a matrix on the TFT array substrate 1 and the TFT 40 isdisposed in the neighborhood of each crossing portion. A sourceelectrode 23 a of the TFT 40 is connected to the data line 54 while adrain electrode 23 b is linked to a pixel electrode 58 as a transparentelectrode. The gate electrode 20 is connected to a scanning line 52.

[0159] The liquid crystal display device 50 thus configured wasdecreased in percent defective of driving circuit and was mitigated indefects such as irregularities in the luminance of image plane due toimproved performance of TFT array and reduction in degradation ofperformance. More specifically, the driving circuit in a liquid crystaldisplay device having conventional TFT's was 15% in percent defective,whereas the liquid crystal display device 50 in this embodiment wasdecreased to 7% in percent defective. The conventional device was 7% inpercent defective in respect of irregularities in the luminance of imageplane while the liquid crystal display device 50 in this embodiment wasdecreased to 3% in percent defective.

[0160] (EL Display Device)

[0161] Next, an EL display device provided with the TFT's produced bythe above-described method will be stated below. The EL display deviceis equipped with a TFT array substrate which includes a TFT forswitching, a TFT for driving and an EL element in each pixel region.

[0162] As shown in FIG. 14, an EL element 60 has anodes 61 comprising atransparent electrode made of ITO or the like, a luminescent layer 62, ahole injection layer 63 and a cathode 64 made of AlLi or the like, whichare all laminated on a polycrystalline silicon film 11. An aluminumquinolinol complex layer 65 is formed on the underside surface (side ofsubstrate 1). A resin black resist is laid between respective anodes 61.A light intercepting layer 66 is formed by photolithography. Theluminescent layer 62 is formed, e.g., by applying luminescent materialsfor red, green and blue colors in a pattern with an ink jet printdevice. The hole injection layer 63 is formed, for example, by vacuumdeposition of polyvinyl carbazole.

[0163] The El element 60 is made of a polydialkylfluorene derivative inthis embodiment and can be formed of other organic materials such asother polyfluorene materials and polyphenyl vinylene materials, orinorganic materials. The EL element 60 can be produced by spin coat orlike coating methods, vapor deposition and ink jet for forming anextrusion. The producing methods can be suitably determined according tothe materials used.

[0164]FIG. 15 is a circuit diagram of the EL display device. The gateelectrode of a TFT 71 for switching is connected to a gate signal line72 while a drain electrode is connected to a drain signal line 73. Asource electrode is connected to the gate electrode of TFT 74 fordriving. The source electrode of TFT 74 for driving is connected to theanode of EL element 60. The drain electrode is connected to a powersource line 76. Indicated at 75 is a condenser.

[0165] When a pulse signal fed to the gate signal line 72 by a drivingcircuit 77 is applied to the gate electrode of TFT 71 for switching, theTFT 71 for switching is brought to an on state. A drain signal fed to adrain signal line 73 by a driving circuit 78 is applied to the gateelectrode of TFT 74 for driving. Thereby the TFT 74 for driving isbrought to an on state, so that a current is supplied from a powersource line 76 to the EL element 60, thereby allowing the EL element 60to emit light.

[0166] The EL display device thus configured exhibited an alleviateddegree of defects such as irregularities in the luminance of image planeand low image quality due to the improved performance of TFT array andlessened degradation of performance. More specifically, the devicehaving conventional TFT's was 8% in percent defective in respect ofirregularities in the luminance of image plane, whereas the EL device 50in this embodiment was reduced to 2% in percent defective in respect ofthis defect. The performance of TFT was impaired in the case ofswitching operation continued for a long time or repeated a large numberof times, leading to deteriorated image quality, but the defects werelessened from 15% in percent defective in the conventional device to 5%in percent defective in this embodiment.

EMBODIMENT 2

[0167] An embodiment 2 of the invention will be described. Constituentparts having the same function in this embodiment and in the followingembodiments as those described above in the embodiment 1 are indicatedby the same reference numerals in the drawings. The description of thisembodiment on the matters already stated in respect of the embodiment 1will be omitted.

[0168] A method of producing a semiconductor thin film in the embodiment2 is characterized in that in the method of producing a semiconductorthin film in the embodiment 1, laser light beams 7 in the primaryirradiation step is applied at a frequency which is different from thatinvolved in the embodiment 1. More specifically, laser beams 7 arerepeatedly applied to a specified range of region on the substrate aplurality of times in the embodiment 1, whereas laser beams aremodulated so as to accomplish irradiation to the entire surface of thesubstrate at one time (one pulse) and is applied to the specified rangeof region only at one time in the embodiment 2. A preferred intensityrange of laser light beams 7 is the same as in the embodiment 1.

[0169] The size of the large size crystal 14 in the semiconductor thinfilm thus obtained was measured under an interatomic force microscope(ATFM) and a transmission electron microscope (TEM). It was found that asize a in a lengthwise direction in a plan view was 1.6 μm and a size bin a widthwise direction was 0.5 μm (see FIGS. 6 and 7). A seriousdefect was not found in the crystal 14. As described above, it isdesirable to irradiate only one pulse to a single part on the substratein the primary irradiation step from the viewpoint of forming a siliconfilm including a crystal of greater size.

[0170] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device, a mask was positioned withalignment keys such that in view of 1.6 μm size of the large sizecrystal, the end of the gate electrode on the drain side was positionedin the center of the large size crystal, i.e. in the range of 0.8 μmaway from the grain boundary. Consequently a single crystal waspermitted to exist in the range of 0.8 μm away from both sides of theboundary between the LDD area 18 b on the drain side and the channelarea 22, which means the absence of grain boundary.

[0171] The TFT thus obtained exhibited a mobility of 180 cm²/V.s and 95%or more of the initial mobility value after the resistance test, namelyboth improved over the mobility of conventional TFT's. The liquidcrystal display device was 3% in percent defective of the drivingcircuit and 0.8% in percent defective in respect of irregularities inthe luminance of image plane, i.e., both good compared with the levelsin the conventional liquid crystal display device. The EL display devicewas 1% in percent defective in respect of irregularities in theluminance of image plane, and 2% in percent defective in the imagequality, i.e., both good compared with the levels in the conventional ELdisplay device.

EMBODIMENT 3

[0172] An embodiment 3 of the invention will be described. A method ofproducing a semiconductor thin film according to the embodiment 3 isdifferent from the method of the embodiment 1 in that in the embodiment3, the step of forming a heat-dissipating layer 4 and an alignment key 5is carried out without executing the pre-irradiation step, and this stepis done by liftoff.

[0173] More specifically, a resist pattern R is formed by applying aresist to the dehydrogenated amorphous silicon film 3 byphotolithography to cover other portions than portions corresponding tothe heat-dissipating layer 4 and the alignment key 5 (FIG. 16(a)). AnMoW film M is formed by vapor deposition (FIG. 16(b)). Then theheat-dissipating layer 4 and the alignment key 5 are formed by removingthe resist pattern R and the MoW film M on the resist pattern R by aresist release agent to form the heat-dissipating layer 4 and thealignment key 5 (FIG. 16(c)).

[0174] Next, the primary irradiation step is carried out to form apolycrystalline silicon film. Laser light beams 7 are applied by theprimary irradiation step in this embodiment preferably at the same rangeof intensity as in the embodiment 1. The laser intensity is 380 mJ/cm²in this embodiment. The frequency of irradiation to the specific onelocation on the substrate is 8 times (8 pulse). Thereby a large sizecrystal is formed around the heat-dissipating layer 4 (see FIG. 6).Thereafter the heat-dissipating layer 4 is eliminated in the same manneras in the embodiment 1.

[0175] Since the pre-irradiation step is not executed in the embodiment3 as in the embodiment 1, amorphous silicon is laid in an area underwhich the heat-dissipating layer 4 was formed. Consequently anadditional irradiation step may be performed with a laser anneal device(ELA device) to crystallize this area. Since a highly membraneous layeris not required at the area where the heat-dissipating layer 4 wasformed, the laser intensity in the additional irradiation step issufficient even if the intensity is lower than in the primaryirradiation step. An excessively high laser intensity may cause a defectin the large size silicon crystal 14 formed in the primary irradiationstep. Hence it is not preferred. Thus, the laser intensity in theadditional irradiation step of this embodiment is preferably the same asin the pre-irradiation step of the embodiment 1 and is 250 mJ/cm² inthis embodiment. The frequency of irradiation to one location on thesubstrate is 10 times. The frequency of irradiation may be various.Thereby polycrystalline silicon with a small size of 30 nm or less isformed in the area where the heat-dissipating layer 4 was laid.

[0176] The size of the large size crystal 14 in the semiconductor thinfilm thus obtained was measured under an interatomic force microscope(ATFM) and a transmission electron microscope (TEM) and was found to beabout 2 μm. A serious defect was not discovered in the crystal 14. Onehundred crystals 14 were checked to investigate the irregularities ofthe size, and the range of size was 2 μm±4 μm, namely the size was nothighly irregular compared with the range of 1.6 μm±0.8 μm in the case ofirradiation at a single pulse instead of irradiation of laser beams at aplurality of pulses in the primary irradiation step of the embodiment 1.

[0177] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device in this embodiment, an offset areais formed in the following manner instead of forming an LDD area in theion doping step of the embodiment 1.

[0178] First, a resist pattern is formed on the gate electrode 20 andover a region of 2 μm away from both ends of electrode 20. Then, highlyconcentrated phosphorus is injected with the resist used as a mask by anion doping device. As a result, as shown in FIG. 17, a channel area 22exists under the gate electrode 20 and offset areas 18 c, 18 d areformed in a region of 2 μm away from both ends of the channel area 22.Highly doped drain areas are formed at portions not covered with theresist pattern. The highly doped drain areas are a source area 24 and adrain area 17. Thereafter a semiconductor device is produced in the samemanner as in the embodiment 1.

[0179] A mask is positioned with an alignment key 5 such that in view of2 μm size of the large size crystal, the end of the gate electrode 20 onthe drain side coincides with the center of the large size crystal,i.e., in a region of 1 μm away from the grain boundary. As aconsequence, a single crystal exists in the range of 1 μm away from bothsides of the boundary between the offset area 18 d on the drain side andthe channel area 22, which means the absence of grain boundary in theTFT.

[0180] The TFT thus obtained exhibited a mobility of 200 cm²/V.s and 95%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's. The liquidcrystal display device was 2.5% in percent defective of the drivingcircuit and 0.6% in percent defective in respect of irregularities inthe luminance of image plane, namely both good compared with the levelsin the conventional liquid crystal display device. The EL display devicewas 0.7% in percent defective in respect of irregularities in theluminance of image plane, and 1.2% in percent defective of the imagequality, both better than the levels in the conventional EL displaydevice.

EMBODIMENT 4

[0181] An embodiment 4 of the invention will be described. In a methodof producing a semiconductor thin film according to the embodiment 4,the step of forming a heat-dissipating layer 4 and an alignment key 5 iscarried out without executing the pre-irradiation step as in the methodof producing a semiconductor thin film according to the embodiment 3.Further the step of forming a heat-dissipating layer 4 and an alignmentkey 5 is carried out by a photo procedure or an etching procedure.

[0182] That is, on a dehydrogenated amorphous silicon film 3 is formedby vapor deposition or sputtering a film of InTio (ITO), i.e., amaterial which is higher in heat conductivity than the silicon film andwhich allows laser light beams to penetrate therethrough. Aheat-dissipating layer 4 and an alignment key 5 made from an ITO patternin a specified shape are formed by a photo procedure and an etchingprocedure (see FIG. 4).

[0183] Thereafter a primary irradiation step is executed to form apolycrystalline silicon film. In this embodiment, laser beams areapplied to one location on the substrate at an intensity of 360 mJ/cM²at a frequency of 300 times (300 pulses). Thereby a large size crystal14 is formed around the heat-dissipating layer 4 (see FIGS. 6 and 7).Subsequently the heat-dissipating layer 4 is removed in the same manneras in the embodiment 1.

[0184] In this embodiment, a pre-irradiation step is not carried out asin the embodiment 3. Since the heat-dissipating layer 4 is formed of ITOhaving a light transmitting property, the silicon film under theheat-dissipating layer 4 is crystallized by the primary irradiationstep. Thus, an additional irradiation step is not needed as in theembodiment 3, thereby contributing to simplification of the productionprocess.

[0185] The size of the large size crystal 14 in the semiconductor thinfilm thus obtained was measured under an interatomic force microscope(AFM) and a transmission electron microscope (TEM). It was found that asize a in a lengthwise direction was 4 μm and a size b in a widthwisedirection was 0.5 μm (see FIGS. 6 and 7). A significant defect was notfound in the crystal. A hundred crystals 14 were checked to investigatethe irregularities of the size, and the range of size was 4 μm±0.4 μm,namely the size was not markedly irregular compared with the range of1.6 μm±0.8 μm in the case of irradiation at a single pulse in theprimary irradiation step of the embodiment 1.

[0186] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device in this embodiment, a mask forforming a gate was positioned by the alignment key 5 such that the gateelectrode 20 coincided with the center of the large size crystal 14.That is, in view of 4 μm size of the large size crystal 14 in alengthwise direction thereof, the end of the gate electrode 20 on thedrain side was positioned 0.8 μm away from the grain boundary by settingthe length at 2.5 μm in a source-drain direction of the gate electrode20. In conformity therewith, a channel area under the gate electrode 20was given a channel length of 2.5 μm, while LDD areas 18 a, 18 b on bothsides of the channel area 22 were given an LDD length of 0.8 μm, so thata single crystal continuously extends over a region including thechannel area 22 and the LDD areas 18 a, 18 b, thereby bringing about astate in which no grain boundary exists (see FIG. 11).

[0187] The TFT thus obtained exhibited a mobility of 320 cm²/V.s and 97%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT'S. The liquidcrystal display device was 1.5% in percent defective of the drivingcircuit and 0.4% in percent defective in respect of irregularities inthe luminance of image plane, i.e., both good compared with the levelsin the conventional liquid crystal display device. The EL display devicewas 0.5% in percent defective in respect of irregularities in theluminance of image plane, and 1.2% in percent defective of the imagequality, both better than the conventional EL display device.

EMBODIMENT 5

[0188] An embodiment 5 of the invention will be described. In a methodof producing a semiconductor thin film according to the embodiment 5, aheat-dissipating layer 4 and an alignment key 5 are formed by a photoprocedure and an etching procedure instead of being formed by vapordeposition using a mask for forming a pattern as done in the method ofproducing a semiconductor thin film according to the embodiment 1.

[0189] A film of MoW which is higher in heat conductivity than a siliconfilm is formed by vapor deposition or sputtering on the polycrystallinesilicon film made by the pre-irradiation step. A heat-dissipating layer4 and an alignment key 5 made from an MoW pattern in a specified shapeare formed by a photo procedure and an etching procedure (see FIG.8(a)).

[0190] Next, the primary irradiation step is carried out to form apolycrystalline silicon film 11. Laser light beams 7 are appliedpreferably at an intensity of 360 mJ/cm² in this embodiment. Thefrequency of irradiation to one location on the substrate is 300 times(300 pulses). Thereby a large size crystal 14 is formed around theheat-dissipating layer 4. Thereafter the heat-dissipating layer 4 iseliminated in the same manner as in the embodiment 1.

[0191] The size of the large size crystal 14 in the semiconductor thinfilm thus obtained was measured under an interatomic force microscope(AFM) and a transmission electron microscope (TEM). It was found that asize a in a lengthwise direction was 4 μm and a size b in a widthwisedirection was 0.5 μm (see FIGS. 6 and 7). A significant defect was notfound in the crystal.

[0192] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device in this embodiment, an offset areawas formed in the same manner as in the embodiment 3 instead of formingan LDD area in the ion doping step of the embodiment 1 (see FIG. 17).

[0193] In this embodiment, a mask for forming the gate was positioned bythe alignment key 5 such that the gate electrode 20 was positioned inthe center of the large size crystal 14. In view of 4 μm size of thelarge size crystal 14 in a lengthwise direction thereof, the end of thegate electrode 20 on the drain side was positioned 0.8 μm away from thegrain boundary by setting the length at 2.5 μm in a source-draindirection of the gate electrode 20. In conformity therewith, a channelarea under the gate electrode 20 was given a length of 2.5 μm, while LDDareas 18 c, 18 d on both sides of the channel area 22 were given alength of 0.8 μm, so that a single crystal continuously extended over aregion including the channel area 22 and the LDD areas 18 c, 18 d,thereby bringing about a state in which no grain boundary existed.

[0194] The TFT thus obtained exhibited a mobility of 310 cm²/V.s and 97%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's. The liquidcrystal display device was 2% in percent defective of the drivingcircuit and 0.4% in percent defective in respect of irregularities inthe luminance of image plane, i.e., both good compared with the levelsin the conventional liquid crystal display device. The EL display devicewas 0.4% in percent defective in respect of irregularities in theluminance of image plane, and 1% in percent defective of the imagequality, namely both good compared with the levels in the conventionalEL display device.

EMBODIMENT 6

[0195] An embodiment 6 of the invention will be described. A method ofproducing a semiconductor thin film according to the embodiment 6 ischaracterized in that in the embodiment 1, minute empty apertures areformed in an undercoat film formed on a substrate. A silica solutionmainly containing Si, O and an organic solvent is applied to a substrate1 which is being rotated. An alcohol (methanol) is used as the organicsolvent in this embodiment. Then the coated substrate 1 is heat-treated,whereby an undercoat film 2 of SiOx including empty apertures is formed(see FIG. 1). A suitable temperature for heat treatment is in the rangeof 450 to 650° C., preferably 550 to 620° C. for forming more minuteapertures and for mitigating the warping of the substrate. In thisembodiment, a temperature of 600° C. is employed for heat treatment.

[0196] When the solidification of silica is executed at 400° C. as inthe conventional heat treatment, the empty apertures have an averagediameter of about 10 μm. In the case of solidification at 600° C., theaverage diameter of empty apertures is lessened to 2 μm or less. Anaverage diameter of empty apertures is preferably in the range of 0.01to 2 μm, and more preferably in the range of 0.05 to 0.1 μm.

[0197] Subsequently a semiconductor thin film was produced in the samemanner as in the embodiment 5. The size of the large size crystal 14 inthe semiconductor thin film was measured under an interatomic forcemicroscope (ATFM) and a transmission electron microscope (TEM). It wasfound that a size a in a lengthwise direction was 30 μm and a size b ina widthwise direction was 0.5 μm (see FIGS. 6 and 7). A serious defectwas not found in the crystal 14. Small size crystals 15 were 200 μm orless in the size.

[0198] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device of this embodiment, the gateelectrode was formed such that the center of the gate electrodecoincided with the center of the large size silicon crystal, using aphoto mask for patterning the polycrystalline silicon layer and designedto incorporate the channel area, LDD areas, source area and drain areain the large size crystal 14. The length of the gate electrode in asource-drain direction was 4 μm. Thus the n-type TFT thus obtained had achannel length of 4 μm, an LDD length of 1.5 μm, and a source length anddrain length, respectively of 10 μm. The areas continuously extended togive a single crystal which means the absence of grain boundary.

[0199] The TFT thus obtained exhibited a mobility of 380 cm²/V.s and 97%or more of the initial mobility value after the resistance test, namelyboth improved over the mobility of conventional TFT'S. The percentdefective was significantly lowered because the undercoat film had anaverage aperture diameter of 2 μm or less and was markedly decreased inthe average aperture diameter compared with the apertures diameter ofconventional porous undercoat film.

[0200] The liquid crystal display device was 1.5% in percent defectiveof the driving circuit and 0.3% in percent defective in respect ofirregularities in the luminance of image plane, namely both goodcompared with the levels in the conventional liquid crystal displaydevice. The EL display device was 0.3% in percent defective in respectof irregularities in the luminance of image plane, and 0.7% in percentdefective of the image quality, i.e., both good compared with the levelsin the conventional EL display device.

EMBODIMENT 7

[0201] An embodiment 7 of the invention will be described. A method ofproducing a semiconductor thin film according to the embodiment 7 ischaracterized in that in the embodiment 1, an undercoat film is formedon a substrate and another undercoat film having a porous layer isformed.

[0202] In the same manner as in the embodiment 1, an undercoat film 2 ofSiO₂ having a thickness of 300 nm is formed on a substrate 1 by aTEOS-CVD method. Laser beams are applied to a film-forming siliconsubstrate as a target at an intensity at which silicon is vaporized. Asilicon film is formed on the undercoat film 2 by laser ablationpermitting vapor deposition of silicon particles. The obtained siliconfilm contains numerous empty apertures. Then, the silicon film isoxidized. Plasma is generated in an ozone or oxygen atmosphere, wherebythe silicon film containing the empty apertures which is formed by laserablation is oxidized to an SiO₂ film 2 a (see FIG. 18). The SiO₂ film 2a has numerous empty apertures, which have an average diameter of 1 μmor less. Since the SiO₂ film 2 a having empty apertures are unlikely tosufficiently prevent diffusion of glass or like dopants from thesubstrate 1 through the semiconductor layer, the diffusion of glass orlike dopants from the substrate 1 through the semiconductor layer can beprevented without fail by provision of a 2-layer structure composed ofthe undercoat film 2 having a dense SiO₂ layer and the undercoat film 2a having a porous layer.

[0203] Thereafter a semiconductor thin film was produced in the samemanner as in the embodiment 5. The size of the large size crystal 14 inthe semiconductor thin film thus obtained was measured under aninteratomic force microscope (ATFM) and a transmission electronmicroscope (TEM). It was found that a size a in a lengthwise directionwas 30 μm and a size b in a widthwise direction was 0.5 μm (see FIGS. 6and 7). A serious defect was not found in the crystal. The small sizecrystals 15 had a size of 200 μm or less.

[0204] Using the thus obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device of this embodiment, an n-type TFTwas obtained in the same manner as in the embodiment 5, the TFT having achannel length of 4 μm, an LDD length of 1.5 μm, and a source length anddrain length, respectively of 10 μm. The areas continuously extend toform a single crystal which means the absence of grain boundary.

[0205] The TFT thus obtained exhibited a mobility of 380 cm²/V.s and 97%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's.

[0206] The liquid crystal display device was 1.2% in percent defectiveof the driving circuit and 0.2% in percent defective in respect ofirregularities in the luminance of image plane, namely both goodcompared with the levels in the conventional liquid crystal displaydevice. The EL display device was 0.2% in percent defective in respectof irregularities in the luminance of image plane, and 0.5% in percentdefective of the image quality, i.e., both good compared with the levelsin the conventional EL display device.

[0207] The undercoat film having a porous layer can be a porous filmsuch as a spin-on-glass (SOG) film. It was confirmed that a siliconcrystal of large size grows in the porous film. SGO may be eitherorganic or inorganic.

EMBODIMENT 8

[0208] An embodiment 8 of the invention will be described. Methods ofproducing a semiconductor thin film according to the embodiments 8 to 12are characterized in that in the method of producing a semiconductorthin film in the embodiment 1, the primary irradiation step is effectedusing an exposure mask.

[0209] First, an undercoat film 2 and an amorphous silicon film 3 areformed on a substrate 1 in the same manner as in the embodiment 1 (seeFIG. 1). After dehydrogenation is carried out when required, a primaryirradiation step is conducted as follows.

[0210] In this embodiment, an exposure mask 105 is used which comprisesa plate and a plurality of lens 114 in the form of a strip in a planview which are arranged on a plate in parallel with each other, as shownin FIG. 19. The plate constituting the exposure mask 105 may be made ofeither a light transmitting material or a light intercepting material.In this embodiment, quartz having a light transmitting property is used.

[0211] Each lens 114 is formed such that at a side face in a lengthwisedirection as shown in FIG. 20(a), the underside (side opposed to thesubstrate 1) has a concave curved surface 114 a in a circular-arc shapein a side view. The curvature is designed in consideration of locationsof the substrate 1 and the exposure mask 105 so as to bring about aninclining distribution of light quantity passing through the lens 114and applied to the silicon film.

[0212] The exposure mask 105 thus configured was disposed close to thesubstrate 1, and laser beams 7 were applied at one pulse via theexposure mask 105. The intensity of laser beams 7 is preferably in thesame range as in the primary irradiation step of the embodiment 1, andwas 380 mJ/cm² in this embodiment. Thereby laser beams 7 passing throughthe lens 114 brings about an inclining distribution of light quantity ina lengthwise direction of the lens 114 and an inclining temperaturegradient in the same direction as the silicon film 11, as shown in FIG.20(b). As a consequence, the crystal grows from the portions involvingthe smallest light quantity (two portions in FIG. 20(b)) toward the sideof the center of the lens 114 and the side of periphery thereof, finallydeveloping into a large size crystal 14. The size of the large sizecrystal 14 was measured under an interatomic force microscope (ATFM) anda transmission electron microscope (TEM). It was found that a size a ina lengthwise direction was 6 μm and a size b in a widthwise directionwas 2 μm (see FIGS. 6 and 7). A serious defect was not found in thecrystal 14. As described above, a large size silicon crystal 14 isformed in a position corresponding to the lens 114 of the exposure mask105 in this embodiment. In this embodiment, the exposure mask 105 has apattern for forming a key (not shown) by which an alignment key can beformed. A detailed description will be given later about the pattern forforming a key in an embodiment 10 to be described later.

[0213] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In this embodiment,a TFT was formed in the position of the large size silicon crystal usingthe alignment key formed by the foregoing pattern for forming a key.

[0214] The TFT thus obtained exhibited a mobility of 170 cm²/V.s and 75%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's. The liquidcrystal display device was 11% in percent defective of the drivingcircuit and 5% in percent defective in respect of irregularities in theluminance of image plane, i.e., both good compared with the levels inthe conventional liquid crystal display device. The EL display devicewas 5% in percent defective in respect of irregularities in theluminance of image plane, and 11% in percent defective of the imagequality, namely both better than the levels in the conventional ELdisplay device. The EL display device showed a luminance of 400 cd/m² inapplying a voltage of 5V, namely improved compared with 300 cd/m² in theconventional EL display device.

[0215] In producing the semiconductor thin film of this embodiment,laser beams 7 were applied once. When laser beams 7 were applied severaltimes (several pulses) to the substrate and the optical axis in a staticstate, namely in a fixed state of positional relation between them(static irradiation), the defects in the silicon crystal in thepolycrystalline silicon film 11 were alleviated. Especially when laserbeams were irradiated at 10 pulses or more (more preferably 100 pulsesor more), not only the defects of silicon crystal were mitigated butalso the size of crystal was extended and the performance of TFTproduced was improved. When laser beams were applied at a plurality ofpulses (scanning irradiation) such that the irradiation area becameoverlapped by 90% in each application of laser beams with thecorrelative positions of the substrate and optical axis graduallychanged, the size of crystal was extended and the performance of TFT wasimproved by using the exposure mask 105 compared with conventionalscanning irradiation although the defects of the crystal were notsignificantly mitigated compared with the foregoing static irradiation.

[0216] The lens 114 used in the exposure mask in this embodiment takes aconcave shape. It is already confirmed that in the case of a convexlens, a suitable distribution of light quantity can be brought about asis the case with subsequent embodiments wherein an exposure mask havinga lens is used.

EMBODIMENT 9

[0217] An embodiment 9 of the invention will be described. In a methodof producing a semiconductor thin film according to the embodiment 9, anexposure mask 139 used comprises, as shown in FIG. 21, a plate made of alight-impermeable material (e.g. stainless steel) and a plurality ofopenings 138 formed therein. The openings 138 are arranged in a row inwhich the area of openings is stepwise changed. A plurality of such rowsare laid while extending in parallel with each other. In other words,the rate of hole area is stepwise varied along a lengthwise direction ofa strip region including this row. The rate of hole area may be varied,for example, by changing the shape of the opening 138 or a space.

[0218] The diameter of the opening 138 is designed such that adistribution of light quantity is established in the range of 250 mJ/cm²to 380 mJ/cm² in the case of irradiation of laser beams 7 to thesubstrate 1 at an intensity of 380 mJ/cm² via the exposure mask 139.Further, an opening 137 in a specified shape for forming a key patternis formed in the exposure mask 139.

[0219] A semiconductor thin film was produced in the same manner as inthe embodiment 8 with the above-configured exposure mask disposed in thevicinity of the substrate 1 (see FIG. 22(a)). Thereby laser beams 7passing through the openings 138 of the exposure mask 139 are allowed togive rise to a distribution of light described. In a method of producinga semiconductor thin film according to the embodiment 9, an exposuremask 139 used comprises, as shown in FIG. 21, a plate made of alight-impermeable material (e.g. stainless steel) and a plurality ofopenings 138 formed therein. The openings 138 are arranged in a row inwhich the area of openings is stepwise changed. A plurality of such rowsare laid while extending in parallel with each other. In other words,the rate of hole area is stepwise varied along a lengthwise direction ofa strip region including this row. The rate of hole area may be varied,for example, by changing the shape of the opening 138 or a space.

[0220] The diameter of the opening 138 is designed such that adistribution of light quantity is established in the range of 250 mJ/cm²to 380 mJ/cm² in the case of irradiation of laser beams 7 to thesubstrate 1 at an intensity of 380 mJ/cm² via the exposure mask 139.Further, an opening 137 in a specified shape for forming a key patternis formed in the exposure mask 139.

[0221] A semiconductor thin film was produced in the same manner as inthe embodiment 8 with the above-configured exposure mask disposed in thevicinity of the substrate 1 (see FIG. 22(a)). Thereby laser beams 7passing through the openings 138 of the exposure mask 139 are allowed togive rise to a distribution of light quantity along the row of openingsand to bring about an inclining temperature gradient in the samedirection as the silicon film 11, resulting in formation of a large sizesilicon crystal 14 from a low temperature portion to a high temperatureportion. The size of the large size silicon crystal 14 was measuredunder an interatomic force microscope (ATFM) and a transmission electronmicroscope (TEM). It was found that a size a in a lengthwise directionwas 10 μm and a size b in a widthwise direction was 3 μm (see FIGS. 6and 7). A significant defect was not found in the crystal. The largesize silicon crystal 14 is formed in a position corresponding to the rowof openings 138 in the exposure mask 139.

[0222] In this embodiment, an opening 137 for forming a key pattern isformed in the exposure mask 139. A region of polycrystalline siliconfilm corresponding to the shape of the opening 137 is formed byirradiation of laser beams and the periphery thereof is a region ofamorphous silicon film. Consequently the key pattern can be used as thealignment key 5 due to the difference of color between thepolycrystalline silicon and amorphous silicon.

[0223] The alignment key 5 formed of an amorphous silicon film may beformed by producing an exposure mask wherein a key portion singly servesas a non-irradiation portion while the periphery thereof is irradiatedwith laser beams.

[0224] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device, a TFT was formed in the positionof the large size silicon crystal using the alignment key 5 in the samemanner as in the embodiment 1.

[0225] The TFT thus obtained exhibited a mobility of 250 cm²/V.s and 83%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT'S. The liquidcrystal display device was 8% in percent defective of the drivingcircuit and 4% in percent defective in respect of irregularities in theluminance of image plane, namely both good compared with the levels inthe conventional liquid crystal display device. The EL display devicewas 3% in percent defective in respect of irregularities in theluminance of image plane, and 8% in percent defective of the imagequality, namely both good compared with the levels in the conventionalEL display device. The EL display device showed a luminance of 450 cd/m²in applying a voltage of 5V, namely improved compared with the levels inthe conventional EL display device.

EMBODIMENT 10

[0226] An embodiment 10 of the invention will be described. In a methodof producing a semiconductor thin film according to the embodiment 10,an exposure mask 205 used comprises a plate and a plurality of lens 214arranged in array (matrix) on the plate as shown in FIG. 23. The plateconstituting the exposure mask 205 may be made of either alight-transmitting material or a light-intercepting material and wasformed of quartz having a light-transmitting property in thisembodiment.

[0227] Each lens 214 is concave and has the underside (the side opposedto the substrate 1) in a concave form as shown in FIG. 24. The internalwall surface of the concave portion is substantially spherically shaped.The curvature of the lens is designed to give rise to an incliningdistribution of light quantity irradiated to the substrate 1 via theexposure mask at an intensity in the range of 250 to 380 mJ/cm² whenlaser beams are applied at an intensity of 380 mJ/cm². Further akey-forming pattern 240 made of a metal or the like without alight-transmitting property is formed at a part of light-transmittingarea of the exposure mask 205 (see FIG. 23).

[0228] The exposure mask 205 thus configured was disposed close to thesubstrate 1, and a semiconductor thin film was produced in the samemanner as in the embodiment 8 (see FIG. 24(a)). Thereby laser beams 7passing through the lens 214 of the exposure mask 205 are allowed, asshown in FIG. 24(b), to give rise to a distribution of light quantityalong a direction of diameter of lens 214 in the shape of a circle in aplan view, and a large size silicon crystal is formed from a lowtemperature portion toward a high temperature portion. The size of thelarge size crystal 14 was measured under an interatomic force microscope(ATFM) and a transmission electron microscope (TEM). It was found that asize a in a lengthwise direction was 10 μm and a size b in a widthwisedirection was 10 μm (see FIGS. 6 and 7). A serious defect was not foundin the crystal 14. As described above, the distribution of lightquantity was brought about also in a widthwise direction in thisembodiment compared with the embodiment 8, so that the crystal was givena substantially circular shape, resulting in extended area of large sizecrystal 14.

[0229] In this embodiment, an amorphous silicon area is formed in a partof the polycrystalline silicon film 11 by the key-forming pattern 240formed in the exposure mask 205 so that due to the difference betweenthe polycrystalline silicon area formed in the periphery and theamorphous silicon area, the formed pattern can be used as the alignmentkey 5.

[0230] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device, a TFT was formed in the positionof the large size crystal 14 using the alignment key 5 in the samemanner as in the embodiment 1. In this embodiment, the center of thelarge size crystal 14 substantially coincided with a positioncorresponding to the center of lens 214 of the exposure mask 205, sothat the position of the formed large size crystal 14 became definiteand constant. The large size crystal 14 and TFT can be more preciselypositioned by the alignment key 5.

[0231] The thus obtained TFT exhibited a mobility of 370 cm²/V.s and 95%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's. The liquidcrystal display device was 3% in percent defective of the drivingcircuit and 1% in percent defective in respect of irregularities in theluminance of image plane, i.e., both good compared with the levels inthe conventional liquid crystal display device. The EL display devicewas 1% in percent defective in respect of irregularities in theluminance of image plane, and 5% in percent defective of the imagequality, namely both good compared with the levels in the conventionalEL display device. The EL display device showed a luminance of 470 cd/m²in applying a voltage of 5V, namely improved compared with the levels inthe conventional EL display device.

EMBODIMENT 11

[0232] An embodiment 11 of the invention will be described. In a methodof producing a semiconductor thin film according to the embodiment 11,an exposure mask 241 used comprises a plate made of a light-transmittingmaterial (such as quartz) and a plurality of concave portions 242 formedin array on the underside of the plate (side opposed to the substrate).The internal wall of the concave portion 242 is formed in a cylindricalshape. A level difference 241 b is formed between the underside 241 a ofthe mask and the portion 242. A key-forming pattern 240 made of metal orthe like without a light-transmitting property is formed in a part ofthe exposure mask 241.

[0233] As shown in FIG. 26(a), the exposure mask 241 thus configured wasdisposed close to the substrate 1, and a semiconductor thin film wasproduced in the same manner as in the embodiment 8. Thereby laser beams7 passing through the exposure mask 241 are allowed, as shown in FIG.26(b)), to give rise to a distribution of light quantity applied to thesubstrate because of phase shift occurring due to the level difference241(b) in the concave portion 242. In the distribution of lightquantity, the light quantity is the smallest in a position correspondingto the vicinity of the level difference 241 b of the concave portion242. The light quantity is increased toward the center side and theopposed side, respectively, along a diameter direction of concaveportion 242. The size of the concave portion 242 and the height of thelevel difference 241 b are designed to bring about an incliningdistribution in the range of 250 mJ/cm² to 380 mJ/cm² in the case ofirradiation at a laser intensity of 380 mJ/cm². Thereby a large sizesilicon crystal is formed toward a high temperature portion from a lowtemperature portion. In this embodiment, laser beams 7 passing throughthe exposure mask 241 are allowed to give rise to a phase differencedistribution by forming a concave portion 242 on an underside 241 a ofthe exposure mask 241, or optionally by forming a convex portion havinga greater thickness than a peripheral portion.

[0234] The size of the large size crystal 14 was measured under aninteratomic force microscope (ATFM) and a transmission electronmicroscope (TEM). It was found that a size a in a lengthwise directionwas 10μm and a size b in a widthwise direction was 10μm (see FIGS. 6 and7). A serious defect was not found in the crystal 14. As describedabove, a distribution of light quantity was brought about in a widthwisedirection in this embodiment as in the embodiment 10, so that thecrystalline shape became substantially circular, resulting in extendedarea of large size crystal 14.

[0235] In this embodiment, an amorphous silicon area is formed by thekey-forming pattern 240 defined in the exposure mask 241 so that due tothe difference between the polycrystalline silicon area formed on theperiphery and the amorphous silicon area, the formed pattern can be usedas the alignment key 5 (see FIG. 26(a)).

[0236] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device, a TFT was produced in theposition of the large size crystal using the alignment key 5 in the samemanner as in the embodiment 1. In this embodiment, the center of thelarge size crystal 14 substantially coincided with the positioncorresponding to the center of the concave portion 242 of the exposuremask 241, so that the position of the formed large size crystal 14becomes definite and constant. The large size crystal 14 and TFT can bemore precisely positioned by the alignment key 5.

[0237] The thus obtained TFT exhibited a mobility of 410 cm²/V.s and 97%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's. The liquidcrystal display device was 2% in percent defective of the drivingcircuit and 0.7% in percent defective in respect of irregularities inthe luminance of image plane, i.e., both good compared with the levelsin the conventional liquid crystal display device. The EL display devicewas 0.6% in percent defective in respect of irregularities in theluminance of image plane, and 4% in percent defective of the imagequality, namely both good compared with the levels in the conventionalEL display device. The EL display device showed a luminance of 520 cd/m²in applying a voltage of 5V, namely improved compared with the levels inthe conventional EL display device.

EMBODIMENT 12

[0238] An embodiment 12 of the invention will be described. In a methodof producing a semiconductor thin film according to the embodiment 12,an exposure mask 339 used comprises a plate made of a light-impermeablematerial (e.g. stainless steel) and a plurality of openings 338 formedtherein as shown in FIG. 27. The openings 338 are arranged in a radialshape in a manner to stepwise increase the area of the individualopenings 338, and forms a region 350, circular in a plan view, which isable to give rise to a distribution of light quantity. In the region350, the openings 338 are arranged such that the rate of hole area perarea unit is stepwise increased from the center toward the periphery ina diameter direction. The shape and space of the openings 338 aredesigned to give an inclining distribution of light quantity applied tothe substrate 1 via the mask 339 at an intensity of 250 mJ/cm² to 380mJ/cm² in the case of irradiation of laser beams at an intensity of 380mJ/cm². The exposure mask 339 has a plurality of regions 350 set thereonin array (matrix) for bringing about the distribution of light quantity.

[0239] In a region other than the regions 350, the openings 340 eachhaving the same area are arranged at equal intervals all over. Furtheran opening 337 in the specified shape is provided for forming a keypattern.

[0240] The exposure mask 339 thus configured was disposed close to thesubstrate 1, and a semiconductor thin film was produced in the samemanner as in the embodiment 8. Laser beams passing through the openings338 of the regions 350 give rise to a distribution of light quantityincreasing from a position corresponding to the center of the region 350on the substrate 1 toward the periphery thereof in a direction ofdiameter, giving an inclining temperature gradient, and resulting information of a large size silicon crystal 14 from a low temperatureportion to a high temperature portion. The size of the large sizecrystal 14 was measured under an interatomic force microscope (ATFM) anda transmission electron microscope (TEM). It was found that a size a ina lengthwise direction was 10 μm and a size b in a widthwise directionwas 10 μm (see FIGS. 6 and 7). A serious defect was not found in thecrystal 14. As described above, a distribution of light quantity isestablished in a widthwise direction in this embodiment as in theembodiment 10, so that the crystal becomes substantially circular,resulting in extended area of large size crystal 14.

[0241] A portion, which was irradiated with laser beams via the openings340 formed in a region other than the region 350, was made into smallsize crystals 15.

[0242] In this embodiment, a polycrystalline silicon area was formed ina position corresponding to the key-forming opening 337 by thekey-forming opening 337 formed in the exposure mask 339. The irradiatedlaser beams were shut off by the exposure mask 339 in its periphery toform an amorphous silicon area. A pattern corresponding to thekey-forming opening 337 can be used as the alignment key 5 due to thedifference of color between the amorphous silicon area andpolycrystalline silicon area.

[0243] Using the obtained semiconductor thin film, a semiconductordevice, a liquid crystal display device and an EL display device wereproduced in the same manner as in the embodiment 1. In producing a TFTconstituting the semiconductor device, a TFT was formed in the positionof the large size crystal 14 using the alignment key 5 in the samemanner as in the embodiment 1. In this embodiment, the center of thelarge size crystal 14 substantially coincided with the positioncorresponding to the center of the light quantity-distribution-formingregion 350 in the exposure mask, so that the position of the formedlarge size crystal 14 became definite and constant. The large sizecrystal 14 and TFT can be more precisely positioned by the alignment key5.

[0244] The thus obtained TFT exhibited a mobility of 410 cm²/V.s and 97%or more of the initial mobility value after the resistance test, namelyboth good compared with the mobility of conventional TFT's. The liquidcrystal display device was 2% in percent defective of the drivingcircuit and 0.7% in percent defective in respect of irregularities inthe luminance of image plane, i.e., both good compared with the levelsin the conventional liquid crystal display device. The EL display devicewas 0.6% in percent defective in respect of irregularities in theluminance of image plane, and 4% in percent defective of the imagequality, i.e., both good compared with the levels in the conventional ELdisplay device. The EL display device showed a luminance of 520 cd/m² inapplying a voltage of 5V, namely improved compared with the levels inthe conventional EL display device.

EMBODIMENT 13

[0245] An embodiment 13 of the invention will be described. A method ofproducing a semiconductor thin film according to the embodiment 13comprises, as shown in FIG. 28(a), the steps of forming an alignment key5 on a substrate 1, forming an undercoat film 2 such as a nitride filmor an oxide film on the substrate 1 and on the alignment key 5, andforming an amorphous silicon film 3 on the undercoat film 2. Thealignment key 5 is made of a material higher in heat conductivity thanthe amorphous silicon film 3 and can be formed by methods in thepreceding embodiments including vapor deposition using a mask, etchingafter forming the key 5, and forming the key 5 after forming a resistpattern, followed by lifting off.

[0246] Then, laser beams are applied to the amorphous silicon film 3under the same conditions as the primary irradiation step in theembodiment 1. Thereby the alignment key 5 functions as theheat-dissipating layer as shown in FIG. 28(a), and a large size crystal14 is formed in the vicinity of the alignment key 5.

[0247] Thereafter a TTF 40 can be formed in the position of the largesize crystal 14 as shown in FIG. 28(c), using the alignment key 5 in thesame manner as in the method of producing a TFT according to theembodiment 1. In this way, the alignment key 5 can also serve as theheat-dissipating layer, thereby contributing to simplification of themethod of producing a semiconductor device.

[0248] The undercoat film 2 may be composed of two layers, i.e. an upperundercoat film 2 b and a lower undercoat film 2 c as shown in FIG.28(d). Optionally the alignment key 5 may be formed between the upperundercoat film 2 b and the lower undercoat film 2 c. In this case, theupper undercoat film 2 b may be preferably made thinner than the lowerundercoat film 2 c, thereby improving the heat conductivity. Moreover,the upper undercoat film 2 b may be porous and the lower undercoat film2 c may be denser than the porous layer.

EMBODIMENT 14

[0249] An embodiment 14 of the invention will be described. Thestructure of the TFT's in the above-described embodiments are generallycalled a coplanar structure or a positive stagger structure. Further, abottom gate structure or a reverse stagger structure is also available.A TFT with the reverse stagger structure can be produced as follows.

[0250] First, an alignment key 5 is formed on a substrate 1 and anundercoat film 2 is formed. Thereafter a metal film is formed bysupttering and subjected to photolithography using an alignment key 5. Agate electrode 20 patterned in a specified position is formed by dryetching or the like (FIG. 29(a)). A gate insulating film 19 is formed bya TEOS-CVD method or the like. Then an amorphous silicon film 3 isformed by a plasma CVD method or the like. Dehydrogenation is conductedby heat treatment or the like (FIG. 29(b)).

[0251] Thereafter, using the amorphous silicon film 3 as thepolycrystalline silicon film 11 in the same manner as in the embodiment1 by a pre-irradiation step, a heat-dissipating layer 4 made of amaterial higher in heat conductivity than the polycrystalline siliconfilm 11 is formed in the vicinity of the gate electrode 20 (FIG. 29(c)).A large size crystal-is formed in the vicinity of the heat-dissipatinglayer 4 by the primary irradiation step and the heat-dissipating layer 4is removed, whereby a semiconductor thin film is completed. Theheat-dissipating layer 4 can be formed, of course, by other methods setout in the foregoing embodiments. The method of producing a TFT usingthis semiconductor thin film can be carried out in the same manner as inthe embodiment 1. The alignment key 5 can be formed together with thegate electrode 20 at the same time when a photo or etching procedure isconducted after forming a metal film for forming a gate electrode,instead of forming the alignment key 5 between the substrate 1 and theundercoat film 2.

[0252] The embodiments of the invention were described in detail.Nevertheless, it goes without saying that the invention is not limitedto the foregoing embodiments and it can be variously modified withoutdeviation from the scope of the invention.

1-15. (Canceled)
 16. A semiconductor device which is provided with athin film transistor having a polycrystalline semiconductor layer, thesemiconductor layer including a channel area, highly doped drain areaspositioned on both sides of the channel area and LDD or offset areaspositioned between the channel area and the highly doped drain areas,the LDD or offset areas being lower in dopant density than the highlydoped drain areas or being free of dopant; wherein any diameter of thecrystal at least partly existing in the LDD or offset areas is largerthan that of other crystals; wherein the thin film transistor is formedin the vicinity of a pattern in a specified shape which is made of amaterial higher in heat conductivity than the semiconductor layer;wherein the pattern is formed between the substrate and thesemiconductor layer; wherein the pattern is covered with an insulatingundercoat film formed between the substrate and the semiconductor layer;wherein the undercoat film includes an upper undercoat film and a lowerundercoat film and the pattern is laid between the upper undercoat filmand the lower undercoat film; and wherein the upper undercoat film is aporous layer and the lower undercoat film is denser than the porouslayer.
 17. The semiconductor device according to claim 16, wherein theupper undercoat film is thinner in thickness than the lower undercoatfilm.
 18. (Canceled)
 19. A liquid crystal display device which ischaracterized in that the device has pixels which are operated by asupply of a voltage via the semiconductor device of claim
 1. 20. An Eldisplay device which is characterized in that the device has pixelswhich are operated by a supply of a voltage via the semiconductordisplay device of claim
 1. 21-26. (Canceled)
 27. A method of producing asemiconductor thin film which is characterized by comprising the step ofirradiating an amorphous or polycrystalline semiconductor thin filmformed on a substrate with high-intensity light rays or laser beams viaan exposure mask to accomplish crystallization wherein the exposure maskhas a lens member with a curved face on at least a part of the top andunderside surfaces, possesses a key-forming pattern made of alight-intercepting material and bring about an inclining distribution oflight quantity applied to the semiconductor thin film, wherein thesemiconductor thin film is crystallized by the step of applyinghigh-intensity light rays or laser beams via the exposure mask toachieve crystallization, and wherein the alignment key comprising anamorphous or polycrystalline silicon area is formed along withcrystallized semiconductor thin film.
 28. The method according to claim27, wherein the lens member is allowed to assume the form of a strip ora circle in a plan view, and wherein the distribution of light quantityis established in a lengthwise direction of the strip or a direction ofdiameter of the circular form.
 29. The method according to claim 27,wherein the curved surface of the lens member is formed by depressing atleast a part of the top and the back surfaces of the exposure mask.30-32. (Canceled)
 33. A method of producing a semiconductor thin filmwhich is characterized by comprising the step of applying high-intensitylight rays or laser beams to an amorphous or polycrystallinesemiconductor thin film formed on a substrate via an exposure mask toachieve crystallization, wherein the exposure mask is formed of alight-intercepting material having a plurality of openings by which aninclining distribution of light quantity applied to the semiconductorthin film is brought about, wherein the plurality of openings arearranged such that a rate of hole area per area unit is stepwise orcontinuously varied along the lengthwise direction of the strip area,and wherein the distribution is brought about along the lengthwisedirection.
 34. A method of producing a semiconductor thin film which ischaracterized by comprising the step of applying high-intensity lightrays or laser beams to an amorphous or polycrystalline semiconductorthin film formed on a substrate via an exposure mask to achievecrystallization, wherein the exposure mask is formed of alight-intercepting material having a plurality of openings by which aninclining distribution of light quantity applied to the semiconductorthin film is brought about, wherein the plurality of openings arearranged such that a rate of hole area per area unit is stepwise orcontinuously increased in a diameter direction from the center of thecircular area toward the periphery of the circular area, and wherein thedistribution is brought about along the diameter direction. 35.(Canceled)
 36. A method of producing a semiconductor device which ischaracterized by comprising the steps of forming an alignment key on apart of a substrate, forming an amorphous or polycrystallinesemiconductor thin film on the substrate and on the alignment key,irradiating the semiconductor thin film with high-intensity light raysor laser beams for crystallization, and forming a gate electrode film onthe semiconductor thin film, wherein the alignment key is formed of amaterial higher in heat conductivity than the semiconductor thin filmand wherein the alignment key functions as a heat-dissipating layer toform a large diameter crystal in its vicinity, and is used at least in aphoto step for forming a pattern of the gate electrode at a specifiedposition by etching a part of the gate electrode film.
 37. A method ofproducing for producing a semiconductor device which is characterized bycomprising the steps of applying high-intensity light rays or laserbeams to an amorphous semiconductor thin film formed on a substrate viaan exposure mask to accomplish crystallization in a state wherein adistribution of light quantity has been brought about forming anamorphous alignment key according to the distribution of light quantityand forming a gate electrode film on the semiconductor thin film,wherein there is a difference in color between the polycrystallinesilicon area formed on the semiconductor thin film and an alignment keycomprising an amorphous silicon area formed by shutting off a part ofpenetrating light rays with the exposure mask, and wherein the alignmentkey having a color different from the color of the polycrystallinesilicon is used at least in a photo step for forming a pattern of thegate electrode at a specified position by etching a part of the gateelectrode film.
 38. A method for manufacturing a semiconductor devicewhich is characterized by comprising the steps of forming a gateelectrode and an alignment key on a part of a substrate, forming anamorphous or polycrystalline semiconductor thin film on the gateelectrode and the alignment key, forming a heat-dissipating layer from amaterial higher in heat conductivity than the semiconductor thin film ina specified position of the semiconductor thin film using the alignmentkey and irradiating the semiconductor thin film with high-intensitylight rays or laser beams for crystallization.